Add {shutdown, Req} to websocket_init/3 to fail a websocket upgrade

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.

test/websocket_handler_init_shutdown.erl

@@ -0,0 +1,30 @@
+%% Feel free to use, reuse and abuse the code in this file.
+
+-module(websocket_handler_init_shutdown).
+-behaviour(cowboy_http_handler).
+-behaviour(cowboy_http_websocket_handler).
+-export([init/3, handle/2, terminate/2]).
+-export([websocket_init/3, websocket_handle/3,
+	websocket_info/3, websocket_terminate/3]).
+
+init(_Any, _Req, _Opts) ->
+	{upgrade, protocol, cowboy_http_websocket}.
+
+handle(_Req, _State) ->
+	exit(badarg).
+
+terminate(_Req, _State) ->
+	exit(badarg).
+
+websocket_init(_TransportName, Req, _Opts) ->
+	Req2 = cowboy_http_req:reply(403, [], [], Req),
+	{shutdown, Req2}.
+
+websocket_handle(_Frame, _Req, _State) ->
+	exit(badarg).
+
+websocket_info(_Info, _Req, _State) ->
+	exit(badarg).
+
+websocket_terminate(_Reason, _Req, _State) ->
+	exit(badarg).