If valid, returns Some(subprotocol)
(if any was requested), or None
if none was
chosen or offered.
If invalid, the reason why the server's upgrade response could not be accepted.
Returns the response object as received from the server for further inspection.
Represents an upgrade response for a WebSocket upgrade request. Can either be valid, in which case the
chosenSubprotocol
method is valid, or if invalid, theinvalidationReason
method can be used to find out why the upgrade failed.