Fix to prevent loop handler awakening immediately after response sent

If a loop handler sent a response (e.g. cowboy_req:chunked_reply/2,/3)
and then returns {loop, Req, HandlerState, hibernate} it
would have a {cowboy_req, resp_sent} message in its message queue. This
message would cause the process to immediately awaken, so it is flushed
before hibernation.
Merge branch 'fix-loop-hibernate' of git://github.com/fishcakez/cowboy