This change allows application developers to refuse websocket upgrades
by returning {shutdown, Req}. The application can also send a reply
with a custom error before returning from websocket_init/3, otherwise
an error 400 is sent.
Note that right now Cowboy closes the connection immediately. Also note
that neither terminate/3 nor websocket_terminate/3 will be called when
the connection is shutdown by websocket_init/3.
