MEDIUM: http: always reject the "PRI" method

MEDIUM: http: always reject the "PRI" method

This method was reserved for the HTTP/2 connection preface, must never
be used and must be rejected. In normal situations it doesn't happen,
but it may be visible if a TCP frontend has alpn "h2" enabled, and
forwards to an HTTP backend which tries to parse the request. Before
this patch it would pass the wrong request to the backend server, now
it properly returns 400 bad req.

This patch should probably be backported to stable versions.