Ignore response body during WebSocket handshake

[waj]

HTTP response during WebSocket handler is not supposed to have a body. Still some servers might return malformed responses. That is the case of Slack that is returning this:

HTTP/1.1 101 Switching Protocols
upgrade: websocket
connection: upgrade
sec-websocket-accept: HYdFWxnjtSQ2eim/P3Qu/3Fud3s=
date: Wed, 03 Jun 2020 20:58:23 GMT
server: envoy
via: envoy-wss-iad-fg1t
transfer-encoding: chunked

That transfer-encoding header is making the parsing of the response to fail. This PR ensures the body is ignored.