HttpObjectAggregator only set Content-Length is not already set.

Motivation:

HEAD requests will have a Content-Length set that doesn't match the
actual length. So we only want to set Content-Length header if it isn't
already set.

Modifications:

If check around setting the Content-Length.

Result:

A HEAD request will no correctly return the specified Content-Length
instead of the body length.

codec-http/src/main/java/io/netty/handler/codec/http/HttpObjectAggregator.java

@@ -140,10 +140,17 @@ protected void aggregate(FullHttpMessage aggregated, HttpContent content) throws
 
     @Override
     protected void finishAggregation(FullHttpMessage aggregated) throws Exception {
-        // Set the 'Content-Length' header.
-        aggregated.headers().set(
-                HttpHeaderNames.CONTENT_LENGTH,
-                String.valueOf(aggregated.content().readableBytes()));
+        // Set the 'Content-Length' header. If one isn't already set.
+        // This is important as HEAD responses will use a 'Content-Length' header which
+        // does not match the actual body, but the number of bytes that would be
+        // transmitted if a GET would have been used.
+        //
+        // See rfc2616 14.13 Content-Length
+        if (!HttpHeaderUtil.isContentLengthSet(aggregated)) {
+            aggregated.headers().set(
+                    HttpHeaderNames.CONTENT_LENGTH,
+                    String.valueOf(aggregated.content().readableBytes()));
+        }
     }
 
     @Override