MINOR: h1: reject websocket handshake if missing key

If a request is identified as a WebSocket handshake, it must contains a
websocket key header or else it can be reject, following the rfc6455.

A new flag H1_MF_UPG_WEBSOCKET is set on such messages.  For the request
te be identified as a WebSocket handshake, it must contains the headers:
  Connection: upgrade
  Upgrade: websocket

This commit is a compagnon of
"MEDIUM: h1: generate WebSocket key on response if needed" and
"MEDIUM: h1: add a WebSocket key on handshake if needed".

Indeed, it ensures that a WebSocket key is added only from a http/2 side
and not for a http/1 bogus peer.
3 files changed