Add fromHttpRequest to UriComponentsBuilder

Before this change, detection of X-Forwarded-* headers was only built
into ServletUriComponentsBuilder.

This change adds a new method for creating a UriComponentsBuilder from
an existing HttpRequest. This is equivalent to the fromUri method +
X-Forwarded-* header values.

spring-webmvc/src/test/java/org/springframework/web/servlet/support/ServletUriComponentsBuilderTests.java

@@ -41,7 +41,7 @@ public class ServletUriComponentsBuilderTests {
 		this.request = new MockHttpServletRequest();
 		this.request.setScheme("http");
 		this.request.setServerName("localhost");
-		this.request.setServerPort(80);
+		this.request.setServerPort(-1);
 		this.request.setContextPath("/mvc-showcase");
 	}