Avoid duplicate Content-Length headers in Netty client

This commit checks that a "Content-Length" request header isn't already
present before adding one in `Netty4ClientHttpRequestFactory`.
`HttpMessageConverter` implementations can write that request header so
the Netty request factory should only write that value when the header
is missing.

If that header is not written (and since we're not dealing with
the HTTP exchange in a chunked-based fashion), the HTTP client might not
send the request body at all.

Issue: SPR-15241

spring-web/src/main/java/org/springframework/http/client/Netty4ClientHttpRequest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -137,14 +137,14 @@ class Netty4ClientHttpRequest extends AbstractAsyncClientHttpRequest implements
 				HttpVersion.HTTP_1_1, nettyMethod, path, this.body.buffer());
 
 		nettyRequest.headers().set(HttpHeaders.HOST, this.uri.getHost());
-		if (this.body.buffer().readableBytes() > 0) {
-			nettyRequest.headers().set(HttpHeaders.CONTENT_LENGTH, this.body.buffer().readableBytes());
-		}
 		nettyRequest.headers().set(HttpHeaders.CONNECTION, "close");
-
 		for (Map.Entry<String, List<String>> entry : headers.entrySet()) {
 			nettyRequest.headers().add(entry.getKey(), entry.getValue());
 		}
+		if (!nettyRequest.headers().contains(HttpHeaders.CONTENT_LENGTH)
+				&& this.body.buffer().readableBytes() > 0) {
+			nettyRequest.headers().set(HttpHeaders.CONTENT_LENGTH, this.body.buffer().readableBytes());
+		}
 
 		return nettyRequest;
 	}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -85,6 +85,7 @@ public class AbstractMockWebServerTestCase {
 	private MockResponse postRequest(RecordedRequest request, String expectedRequestContent,
 			String location, String contentType, byte[] responseBody) {
 
+		assertEquals(1, request.getHeaders().values("Content-Length").size());
 		assertTrue("Invalid request content-length",
 				Integer.parseInt(request.getHeader("Content-Length")) > 0);
 		String requestContentType = request.getHeader("Content-Type");