MEDIUM: h1: generate WebSocket key on response if needed

MEDIUM: h1: generate WebSocket key on response if needed

Add the Sec-Websocket-Accept header on a websocket handshake response.
This header may be missing if a h2 server is used with a h1 client.

The response key is calculated following the rfc6455. For this, the
handshake request key must be stored in the h1 session, as a new field
name ws_key. Note that this is only done if the message has been
prealably identified as a Websocket handshake request.