If client sent no subprotocol (e.g. no Sec-WebSocket-Protocol),
websocket server responded with the following header
  HTTP/1.1 101 Switching Protocols
  Upgrade: websocket
  Connection: Upgrade
  Sec-WebSocket-Accept: E7SRWRnZL9RuGFLuZ0j4508nqdg=
  Sec-WebSocket-Protocol:
So, client may close the connection because it contains
wrong empty Sec-WebSocket-Protocol header.

If client didn't offer any subprotocol, don't set
config.Protocol, so that not emit empty Sec-WebSocket-Protocol.

Fixes golang/go#5457.

R=golang-dev, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/9379044