WebSocket: Subprotocol incorrectly treated as mandatory

[ting-fx]

Describe the bug
The current WebSocket implementation incorrectly requires a subprotocol (Sec-WebSocket-Protocol) during the opening handshake. If no subprotocol is specified, the WebSocket connection fails. According to RFC 6455, subprotocol negotiation is optional.

[fdesbiens]

Thank you for raising this, @ting-fx

One question: I am no Websocket expert. So, what would be the proper mitigation for this issue if our implementation somehow depends on that specific subprotocol to be used? Long-term, I would like to address this, but realistically, we do not have the resources right now.