Include port info in HTTP headers for Netty client

This commit adds the port information in the "Host" HTTP request header
for the Netty-based HTTP client.

Issue: SPR-15706
(cherry-picked from commit baaa84e90)

spring-web/src/test/java/org/springframework/http/client/AbstractMockWebServerTestCase.java

@@ -48,6 +48,8 @@ public class AbstractMockWebServerTestCase {
 		public MockResponse dispatch(RecordedRequest request) throws InterruptedException {
 			try {
 				if (request.getPath().equals("/echo")) {
+					assertThat(request.getHeader("Host"),
+							Matchers.containsString("localhost:" + port));
 					MockResponse response = new MockResponse()
 							.setHeaders(request.getHeaders())
 							.setHeader("Content-Length", request.getBody().size())