code: public ChannelFuture handshake(Channel channel, HttpRequest req) {
          return handshake(channel, req, null, channel.newPromise());
      }
docs: Performs the opening handshake. When call this method you <strong>MUST NOT</strong> retain the
  {@link FullHttpRequest} which is passed in.
  
  @param channel
  Channel
  @param req
  HTTP Request
  @return future
  The {@link ChannelFuture} which is notified once the opening handshake completes