Allow setting `Sec-WebSocket-Protocol` response header

[lehmanju]

As the title says, when accepting a new connection, allow setting custom response headers such as "Sec-WebSocket-Protocol".

[Gelbpunkt]

That seems sensible and should be a trivial addition. I don't have the time to implement this immediately, but I'd be happy to review a small PR for it. Otherwise I'll probably get to this in a few weeks.