Backport changes that incidentally fix WSClient sending HTTP CONNECT to target host

see https://github.com/marcospereira/async-http-client/commit/d5960563dcbd09854b8ae3e416108ce770b292c2
and #1274

Author: slandelle

Diff for: client/src/main/java/org/asynchttpclient/netty/handler/AsyncHttpClientHandler.java

@@ -67,8 +67,10 @@ public void channelRead(final ChannelHandlerContext ctx, Object msg) throws Exce
         Object attribute = Channels.getAttribute(channel);
 
         try {
-            if (attribute instanceof OnLastHttpContentCallback && msg instanceof LastHttpContent) {
-                ((OnLastHttpContentCallback) attribute).call();
+            if (attribute instanceof OnLastHttpContentCallback) {
+                if (msg instanceof LastHttpContent) {
+                    ((OnLastHttpContentCallback) attribute).call();
+                }
 
             } else if (attribute instanceof NettyResponseFuture) {
                 NettyResponseFuture<?> future = (NettyResponseFuture<?>) attribute;