Handle context-path on X-Forwarded-Prefix support

Since revision 7b270c4b46, Zuul add support of `X-Forwarded-Prefix` for route with `stripPrefix=true`.
However it supposes that Zuul context-path is equals to `/`. By using a custom context-path on Zuul we can expect that `zuul.add-proxy-headers=true` will compute a prefix regarding that context-path.

Indeed with following architecture:

- Zuul with context-path equals to `/foo`
- A random service (*bar-service*) without context-path

Zuul configurations:

```
zuul:
  add-proxy-headers: true
  routes:
    bar-service: /bar/**
```

We expect that *bar-service* when targeting `http://blabla.com/foo/bar/1`  will produce endpoint url equals to `http://blabla.com/foo/bar` and not only `http://blabla.com/bar` (that will not be resolvable).

Furthermore context-path influence not only `stripPrefix=true` routes because with following configuration

```
zuul:
  add-proxy-headers: true
  routes:
    bar-service:
      path: /bar/**
      strip-prefix: false
```

We expect that *bar-service* when targeting `http://blabla.com/foo/bar/1` will also prodcues endpoint url equals to `http://blabla.com/foo/bar`...

---

Moreover since *Spring 4.3* and new [`ForwardedHeaderFilter`](http://docs.spring.io/spring-framework/docs/4.3.0.BUILD-SNAPSHOT/javadoc-api/org/springframework/web/filter/ForwardedHeaderFilter.html) to handle `X-Forwarded-*` headers, when filter catches `X-Forwarded-Prefix` header it will override request context-path by `X-Forwarded-Prefix` header value and then **removes headers from request**. Thus the fact to support context-path will allow to use that filter with Zuul!

---

Attention there is only one edge case, when `X-Forwarded-Prefix` header is present as same as custom `context-path`! In that case context-path will be ignored in favor to `X-Forwarded-Prefix` as same does `ForwardedHeaderFilter`

spring-cloud-netflix-core/src/main/java/org/springframework/cloud/netflix/zuul/filters/pre/PreDecorationFilter.java

@@ -121,23 +121,29 @@ public class PreDecorationFilter extends ZuulFilter {
 							String.valueOf(ctx.getRequest().getServerPort()));
 					ctx.addZuulRequestHeader(ZuulHeaders.X_FORWARDED_PROTO,
 							ctx.getRequest().getScheme());
+					String forwardedPrefix =
+							ctx.getRequest().getHeader("X-Forwarded-Prefix");
+					String contextPath = ctx.getRequest().getContextPath();
+					String prefix = StringUtils.hasLength(forwardedPrefix)
+							? forwardedPrefix
+							: (StringUtils.hasLength(contextPath) ? contextPath : null);
 					if (StringUtils.hasText(route.getPrefix())) {
-						String existingPrefix = ctx.getRequest()
-								.getHeader("X-Forwarded-Prefix");
 						StringBuilder newPrefixBuilder = new StringBuilder();
-						if (StringUtils.hasLength(existingPrefix)) {
-							if (existingPrefix.endsWith("/")
+						if (prefix != null) {
+							if (prefix.endsWith("/")
 									&& route.getPrefix().startsWith("/")) {
-								newPrefixBuilder.append(existingPrefix, 0,
-										existingPrefix.length() - 1);
+								newPrefixBuilder.append(prefix, 0,
+										prefix.length() - 1);
 							}
 							else {
-								newPrefixBuilder.append(existingPrefix);
+								newPrefixBuilder.append(prefix);
 							}
 						}
 						newPrefixBuilder.append(route.getPrefix());
-						ctx.addZuulRequestHeader("X-Forwarded-Prefix",
-								newPrefixBuilder.toString());
+						prefix = newPrefixBuilder.toString();
+					}
+					if (prefix != null) {
+						ctx.addZuulRequestHeader("X-Forwarded-Prefix", prefix);
 					}
 					String xforwardedfor = ctx.getRequest().getHeader("X-Forwarded-For");
 					String remoteAddr = ctx.getRequest().getRemoteAddr();

spring-cloud-netflix-core/src/test/java/org/springframework/cloud/netflix/zuul/filters/pre/PreDecorationFilterTests.java

@@ -136,6 +136,86 @@ public class PreDecorationFilterTests {
 				getHeader(ctx.getOriginResponseHeaders(), "x-zuul-serviceid"));
 	}
 
+	@Test
+	public void routeWithContextPath() {
+		this.properties.setStripPrefix(false);
+		this.request.setRequestURI("/api/foo/1");
+		this.request.setContextPath("/context-path");
+		this.routeLocator.addRoute(
+				new ZuulRoute("foo", "/api/foo/**", "foo", null, false, null, null));
+		this.filter.run();
+		RequestContext ctx = RequestContext.getCurrentContext();
+		assertEquals("/api/foo/1", ctx.get("requestURI"));
+		assertEquals("localhost", ctx.getZuulRequestHeaders().get("x-forwarded-host"));
+		assertEquals("80", ctx.getZuulRequestHeaders().get("x-forwarded-port"));
+		assertEquals("http", ctx.getZuulRequestHeaders().get("x-forwarded-proto"));
+		assertEquals("/context-path",
+				ctx.getZuulRequestHeaders().get("x-forwarded-prefix"));
+		assertEquals("foo",
+				getHeader(ctx.getOriginResponseHeaders(), "x-zuul-serviceid"));
+	}
+
+	@Test
+	public void prefixRouteWithContextPath() {
+		this.properties.setPrefix("/api");
+		this.properties.setStripPrefix(true);
+		this.request.setRequestURI("/api/foo/1");
+		this.request.setContextPath("/context-path");
+		this.routeLocator.addRoute(
+				new ZuulRoute("foo", "/foo/**", "foo", null, false, null, null));
+		this.filter.run();
+		RequestContext ctx = RequestContext.getCurrentContext();
+		assertEquals("/foo/1", ctx.get("requestURI"));
+		assertEquals("localhost", ctx.getZuulRequestHeaders().get("x-forwarded-host"));
+		assertEquals("80", ctx.getZuulRequestHeaders().get("x-forwarded-port"));
+		assertEquals("http", ctx.getZuulRequestHeaders().get("x-forwarded-proto"));
+		assertEquals("/context-path/api",
+				ctx.getZuulRequestHeaders().get("x-forwarded-prefix"));
+		assertEquals("foo",
+				getHeader(ctx.getOriginResponseHeaders(), "x-zuul-serviceid"));
+	}
+
+	@Test
+	public void routeIgnoreContextPathIfPrefixHeader() {
+		this.properties.setStripPrefix(false);
+		this.request.setRequestURI("/api/foo/1");
+		this.request.setContextPath("/context-path");
+		this.request.addHeader("X-Forwarded-Prefix", "/prefix");
+		this.routeLocator.addRoute(
+				new ZuulRoute("foo", "/api/foo/**", "foo", null, false, null, null));
+		this.filter.run();
+		RequestContext ctx = RequestContext.getCurrentContext();
+		assertEquals("/api/foo/1", ctx.get("requestURI"));
+		assertEquals("localhost", ctx.getZuulRequestHeaders().get("x-forwarded-host"));
+		assertEquals("80", ctx.getZuulRequestHeaders().get("x-forwarded-port"));
+		assertEquals("http", ctx.getZuulRequestHeaders().get("x-forwarded-proto"));
+		assertEquals("/prefix",
+				ctx.getZuulRequestHeaders().get("x-forwarded-prefix"));
+		assertEquals("foo",
+				getHeader(ctx.getOriginResponseHeaders(), "x-zuul-serviceid"));
+	}
+
+	@Test
+	public void prefixRouteIgnoreContextPathIfPrefixHeader() {
+		this.properties.setPrefix("/api");
+		this.properties.setStripPrefix(true);
+		this.request.setRequestURI("/api/foo/1");
+		this.request.setContextPath("/context-path");
+		this.request.addHeader("X-Forwarded-Prefix", "/prefix");
+		this.routeLocator.addRoute(
+				new ZuulRoute("foo", "/foo/**", "foo", null, false, null, null));
+		this.filter.run();
+		RequestContext ctx = RequestContext.getCurrentContext();
+		assertEquals("/foo/1", ctx.get("requestURI"));
+		assertEquals("localhost", ctx.getZuulRequestHeaders().get("x-forwarded-host"));
+		assertEquals("80", ctx.getZuulRequestHeaders().get("x-forwarded-port"));
+		assertEquals("http", ctx.getZuulRequestHeaders().get("x-forwarded-proto"));
+		assertEquals("/prefix/api",
+				ctx.getZuulRequestHeaders().get("x-forwarded-prefix"));
+		assertEquals("foo",
+				getHeader(ctx.getOriginResponseHeaders(), "x-zuul-serviceid"));
+	}
+
 	@Test
 	public void forwardRouteAddsLocation() throws Exception {
 		this.properties.setPrefix("/api");
@@ -377,7 +457,7 @@ public class PreDecorationFilterTests {
 		assertTrue("sensitiveHeaders is wrong: " + sensitiveHeaders,
 				sensitiveHeaders.containsAll(Arrays.asList("x-bar", "x-foo")));
 	}
-	
+
 	@Test
 	public void urlProperlyDecodedWhenCharacterEncodingIsSet() throws Exception {
 		this.request.setCharacterEncoding("UTF-8");