Re-introduce writeHeaders() in ServerHttpResponse

This commit brings back the writeHeaders method on ServerHttpResponse
that was once added (2a6a4f) and then removed (9c7151).

This version is a little simpler since writeHeaders doesn't explicitly
flush/send headers which runtimes are expected to do by default.
Instead the main purpose of writeHeaders now is to ensure changes made
via HttpHeaders are applied to the underlying runtime response at some
point and we now do that once at the very end.

This approach provides the most flexibility (vs keeping HttpHeaders in
sync) because it allows a full and consistent set of mutative
operations for both headers and cookies (to be added) regardless of the
API exposed by the underlying runtime.

spring-web-reactive/src/test/java/org/springframework/http/server/reactive/MockServerHttpResponse.java

@@ -60,4 +60,8 @@ public class MockServerHttpResponse implements ServerHttpResponse {
 		return this.body;
 	}
 
+	@Override
+	public void writeHeaders() {
+	}
+
 }

spring-web-reactive/src/test/java/org/springframework/web/reactive/handler/SimpleUrlHandlerMappingIntegrationTests.java

@@ -32,8 +32,6 @@ import org.springframework.http.RequestEntity;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.server.reactive.AbstractHttpHandlerIntegrationTests;
 import org.springframework.http.server.reactive.HttpHandler;
-import org.springframework.http.server.reactive.ServerHttpRequest;
-import org.springframework.http.server.reactive.ServerHttpResponse;
 import org.springframework.web.client.HttpClientErrorException;
 import org.springframework.web.client.RestTemplate;
 import org.springframework.web.reactive.DispatcherHandler;