server handshake handler wrongly parse subprotocol and using wrong logic to generate subprotocol response

[jangko]

according to https://datatracker.ietf.org/doc/html/rfc6455#section-4.2.2:

• Sec-WebSocket-Protocol field received from client will contains a list of strings, not a single string. Currently, the server is doing single string comparison, not a list of strings comparison.

• if the server agree to one of sub protocols offered by client, it must return that sub protocol name as value. Currently, the server only check if it support the sub protocol or not when constructing sub protocol response(Sec-WebSocket-Protocol field). The server must also check if that sub protocol is already agreed.

[jangko]

The client if suffer from the same validation, see #24

[jangko]

fixed by #29. close