Fix remote context handling in UrlTag

Prior to this change, the `UrlTag` would simply append the remote
context and the path value in case of a context relative URL.

The following code snippet would output "//foo":

```
<spring:url value="/foo" context="/" />
```

This change now removes trailing slashes in remote context to avoid
this.

Issue: SPR-12782

spring-webmvc/src/test/java/org/springframework/web/servlet/tags/UrlTagTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2013 the original author or authors.
+ * Copyright 2002-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -528,6 +528,20 @@ public class UrlTagTests extends AbstractTagTests {
 		assertEquals("/some-other-context/url/path", uri);
 	}
 
+	public void testCreateUrlRemoteContextSingleSlash() throws JspException {
+		((MockHttpServletRequest) context.getRequest())
+				.setContextPath("/app-context");
+
+		tag.setValue("/url/path");
+		tag.setContext("/");
+
+		tag.doStartTag();
+
+		String uri = invokeCreateUrl(tag);
+
+		assertEquals("/url/path", uri);
+	}
+
 	public void testCreateUrlWithParams() throws JspException {
 		tag.setValue("url/path");