do not ignore data received immediately after switching to raw

[chilikk]

When an HTTP connection is upgraded to raw it may happen that the body/the data belonging to the raw connection arrives in the same message as HTTP response and headers from the TCP socket. This change fixes the bug where such data has been dropped. With this fix the data following the HTTP response is forwarded to the gun_raw handler.

[essen]

Hello!

Probably better to change the switch_protocol tuple to have the data? All other switch_protocol can be updated to have <<>> by default and do nothing.

[chilikk]

Changed according to your suggestion. I'm not exactly sure how to handle all switch_protocol: the same handling could possibly apply to gun_http:handle_connect case, but I'm unsure what would be the scenario and how to test it, so I didn't change. Also, in gun_tunnel I don't handle any other case than <<>>, because my understanding is that gun_tunnel is only used from HTTP/2 and this situation would not be possible there due to the framing mechanism.