Add {switch_handler, Module} return value to cowboy_rest

Also {switch_handler, Module, Opts}.

Allows switching to a different handler type. This is
particularly useful for processing most of the request
with cowboy_rest and then streaming the response body
using cowboy_loop.

test/handlers/switch_handler_h.erl

@@ -0,0 +1,36 @@
+-module(switch_handler_h).
+
+-export([init/2]).
+-export([content_types_provided/2]).
+-export([provide/2]).
+-export([info/3]).
+
+init(Req, State) ->
+	{cowboy_rest, Req, State}.
+
+content_types_provided(Req, State) ->
+	{[{<<"text/plain">>, provide}], Req, State}.
+
+provide(Req0, run) ->
+	Req = cowboy_req:stream_reply(200, Req0),
+	send_after(0),
+	{{switch_handler, cowboy_loop}, Req, 0};
+provide(Req0, hibernate) ->
+	Req = cowboy_req:stream_reply(200, Req0),
+	send_after(0),
+	{{switch_handler, cowboy_loop, hibernate}, Req, 0}.
+
+send_after(N) ->
+	erlang:send_after(100, self(), {stream, msg(N)}).
+
+msg(0) -> <<"Hello\n">>;
+msg(1) -> <<"streamed\n">>;
+msg(2) -> <<"world!\n">>;
+msg(3) -> stop.
+
+info({stream, stop}, Req, State) ->
+	{stop, Req, State};
+info({stream, What}, Req, State) ->
+	cowboy_req:stream_body(What, nofin, Req),
+	send_after(State + 1),
+	{ok, Req, State + 1}.