I've seen implementations in the wild that send 'Upgrade: WebSocket',
which currently fails the handshake.
From https://datatracker.ietf.org/doc/html/rfc6455:
"If the response lacks an |Upgrade| header field or the |Upgrade| header
field contains a value that is not an ASCII case-insensitive match for
the value "websocket", the client MUST _Fail the WebSocket Connection_."