User defined Content-Length request header should be honored, close #1440

Motivation:

When Content-Length is known beforehand, we shouldn’t ignore it and
enforce chunked transfer-encoding.

Modification:

Honor Content-Length request header when it's defined.

Result:

Chunked transfer-encoding is no longer enforced

Author: slandelle

client/src/main/java/org/asynchttpclient/netty/request/NettyRequestFactory.java

@@ -177,10 +177,12 @@ public NettyRequest newNettyRequest(Request request, boolean forceConnect, Proxy
         }
 
         if (body != null) {
-            if (body.getContentLength() < 0) {
-                headers.set(TRANSFER_ENCODING, HttpHeaderValues.CHUNKED);
-            } else {
-                headers.set(CONTENT_LENGTH, body.getContentLength());
+            if (!headers.contains(CONTENT_LENGTH)) {
+                if (body.getContentLength() < 0) {
+                    headers.set(TRANSFER_ENCODING, HttpHeaderValues.CHUNKED);
+                } else {
+                    headers.set(CONTENT_LENGTH, body.getContentLength());
+                }
             }
 
             if (body.getContentTypeOverride() != null) {