Various improvements to web.reactive.function

This commit introduces the following changes to web.reactive.function:

- Added RouterFunction.andRoute(), a combination of RouterFunction.and()
with RouterFunctions.route()
- ServerRequest.pathVariable() returns String instead of
Optional<String>. An exception is thrown if the variable is not present.
- Added HandlerFilterFunction.andThen and HandlerFilterFunction.apply()

spring-web-reactive/src/test/java/org/springframework/web/reactive/function/DefaultServerRequestTests.java

@@ -119,7 +119,15 @@ public class DefaultServerRequestTests {
 		Map<String, String> pathVariables = Collections.singletonMap("foo", "bar");
 		when(mockExchange.getAttribute(RouterFunctions.URI_TEMPLATE_VARIABLES_ATTRIBUTE)).thenReturn(Optional.of(pathVariables));
 
-		assertEquals(Optional.of("bar"), defaultRequest.pathVariable("foo"));
+		assertEquals("bar", defaultRequest.pathVariable("foo"));
+	}
+
+	@Test(expected = IllegalArgumentException.class)
+	public void pathVariableNotFound() throws Exception {
+		Map<String, String> pathVariables = Collections.singletonMap("foo", "bar");
+		when(mockExchange.getAttribute(RouterFunctions.URI_TEMPLATE_VARIABLES_ATTRIBUTE)).thenReturn(Optional.of(pathVariables));
+
+		assertEquals("bar", defaultRequest.pathVariable("baz"));
 	}
 
 	@Test

spring-web-reactive/src/test/java/org/springframework/web/reactive/function/support/ServerRequestWrapperTests.java

@@ -116,9 +116,9 @@ public class ServerRequestWrapperTests {
 	public void pathVariable() throws Exception {
 		String name = "foo";
 		String value = "bar";
-		when(mockRequest.pathVariable(name)).thenReturn(Optional.of(value));
+		when(mockRequest.pathVariable(name)).thenReturn(value);
 
-		assertEquals(Optional.of(value), wrapper.pathVariable(name));
+		assertEquals(value, wrapper.pathVariable(name));
 	}
 
 	@Test