Fix loop handler keepalive race condition

Previously if a loop handler received the timeout message from a
previous request on the same connection the socket would be set to
{active, once} incorrectly - when a socket packet was already in the
message queue. This second packet would not be added to the buffer
before a Handler:info/3 call if a user message was in the message
queue before both socket packets.

test/http_SUITE_data/http_loop_stream_recv.erl

@@ -0,0 +1,41 @@
+%% Feel free to use, reuse and abuse the code in this file.
+
+-module(http_loop_stream_recv).
+-export([init/3]).
+-export([info/3]).
+-export([terminate/3]).
+
+init({_, http}, Req, _) ->
+	receive after 100 -> ok end,
+	self() ! stream,
+	{loop, Req, 1, 100}.
+
+info(stream, Req, Id) ->
+	case stream_next(Req) of
+		{ok, Id, Req2} ->
+			info(stream, Req2, Id+1);
+		{done, Req2} ->
+			{ok, Req3} = cowboy_req:reply(200, Req2),
+			{ok, Req3, Id}
+	end.
+
+terminate({normal, shutdown}, _, _) ->
+	ok.
+
+stream_next(Req) ->
+	stream_next(<<>>, Req).
+
+stream_next(Buffer, Req) ->
+	case cowboy_req:stream_body(Req) of
+		{ok, Packet, Req2} ->
+			case <<Buffer/binary, Packet/binary>> of
+				<<Id:32>> ->
+					{ok, Id, Req2};
+				Buffer2 when byte_size(Buffer2) < 4 ->
+					stream_next(Buffer2, Req2);
+				_InvalidBuffer ->
+					{error, invalid_chunk}
+			end;
+		Other ->
+			Other
+	end.