Return no protocol instead of erroring when not agreeing on a protocol

[flying-sheep]

sorry, my implementation of websocket subprotocols has a mistake, i.e. the spec says:

The server selects one or none of the acceptable protocols and echoes that value in its handshake to indicate that it has selected that protocol.

so instead of a Bad Request exception, we should send back no Sec-WebSocket-Protocol header at all and leave it to the client to drop the connection or carry on without protocol (however dumb that sounds).

[asvetlov]

Sounds good.
Would you make a pull request?

[flying-sheep]

gladly. i see that this project doesn’t use logging: wouldn’t it be useful to introduce it? then i could replace this with a warning, but it doesn’t make sense if we don’t log anything else.

/edit: i’ll just use warnings.warn.

[lock]

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