websocket: return an error HTTP response for bad websocket request.

websocket: return an error HTTP response for bad websocket request.

websocket spec had changed server-side requiements to return
an HTTP response with an appropriate error code (such as 400 Bad
Request) when it finds client did not send a handshake that matches
websocket protocol, rather than just closing connection.
It needs to flush out response before closing connection.
Fixes issues 2396.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5318072