Keep Host header in the outbound request (#1412)

Commit 0d2b87a changed how outbound request headers are updated, instead of using
HttpClientOperations.headers() method to merged headers, HttpHeaders.add() is now used.

However HttpClientOperations.headers() method treats Host header
specially and do not override it from the provided headers collection.

This fix just copies the same logic within NettyRoutingFilter for
backward compatibility.

There is one issue with this approach, if Host header is set by a filter
it will be ignored. However this is how it worked before, with
HttpClientOperations.headers() method in place.

gh-1345

spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/filter/NettyRoutingFilter.java

@@ -122,6 +122,10 @@ public class NettyRoutingFilter implements GlobalFilter, Ordered {
 				String host = request.getHeaders().getFirst(HttpHeaders.HOST);
 				headers.add(HttpHeaders.HOST, host);
 			}
+			else {
+				// let Netty set it based on hostname
+				headers.remove(HttpHeaders.HOST);
+			}
 		}).request(method).uri(url).send((req, nettyOutbound) -> {
 			if (log.isTraceEnabled()) {
 				nettyOutbound.withConnection(connection -> log.trace(

spring-cloud-gateway-core/src/test/java/org/springframework/cloud/gateway/filter/NettyRoutingFilterIntegrationTests.java

@@ -27,6 +27,7 @@ import org.springframework.context.annotation.Import;
 import org.springframework.http.HttpStatus;
 import org.springframework.test.annotation.DirtiesContext;
 import org.springframework.test.context.junit4.SpringRunner;
+import org.springframework.test.web.reactive.server.WebTestClient;
 
 import static org.springframework.boot.test.context.SpringBootTest.WebEnvironment.RANDOM_PORT;
 
@@ -46,6 +47,18 @@ public class NettyRoutingFilterIntegrationTests extends BaseWebClientTests {
 				.isEqualTo("Response took longer than timeout: PT3S");
 	}
 
+	@Test
+	public void outboundHostHeaderNotOverwrittenByInbound() {
+		// different base url to have different host header in inbound / outbound requests
+		// Host: 127.0.0.1 -> request to Gateway, Host: localhost -> request from Gateway,
+		// resolved from lb://testservice
+		WebTestClient client = testClient.mutate().baseUrl("http://127.0.0.1:" + port)
+				.build();
+
+		client.get().uri("/headers").exchange().expectBody().jsonPath("$.headers.host")
+				.isEqualTo("localhost:" + port);
+	}
+
 	@EnableAutoConfiguration
 	@SpringBootConfiguration
 	@Import(DefaultTestConfig.class)