websocket: tweak hybi ReadHandshake to supports Firefox implementation
Firefox Websocket implementation send a "Connection: keep-alive, upgrade"
header during the handshake (and as descripted on the last hybi draft
the "Connection" header must include, but doesn't need to be equal to,
"upgrade":
'4. A "Connection" header field that includes the token "Upgrade",
treated as an ASCII case-insensitive value.'
From:
http://tools.ietf.org/html/draft-ietf-hybi-thewebsocketprotocol-17#page-23
R=golang-dev, ukai, cw, rsc
CC=golang-dev
https://golang.org/cl/
5233059