HTTP/2: handle 'PRI' method found in HTTP/1.x traffic

HTTP/2: handle 'PRI' method found in HTTP/1.x traffic

draft-ietf-httpbis-http2-16 section 11.6 registers the method PRI.
"
  This method is never used by an actual client.
  This method will appear to be used when an HTTP/1.1 server or
  intermediary attempts to parse an HTTP/2 connection preface.
"

If seen with a non-2.0 version number it means some client or proxy has
mishandled an HTTP/2.0 connection preface and corrupted the traffic.