Issue #4376 Async Content Complete

If completion has to do a flush, then we need a call to closed to
avoid leaking buffers.

Signed-off-by: Greg Wilkins <gregw@webtide.com>

jetty-server/src/main/java/org/eclipse/jetty/server/HttpChannelState.java

@@ -968,6 +968,9 @@ protected void completed()
             }
         }
 
+        // release any aggregate buffer from a closing flush
+        _channel.getResponse().getHttpOutput().closed();
+
         if (event != null)
         {
             cancelTimeout(event);