Support multiple subprotocols in Websocket handshake for client side

[RomanBrodetski]

Currently passing multiple websockets subprotocols in Sec-WebSocket-Protocol header is not supported (see #2606). This PR makes it possible to do so, passing them separated by comma in subprotocol: Option[String] field of the WebSocketRequest model. This corresponds to the Sec-WebSocket-Protocol header definition in RFC 6455

A better solution would be to change the WebSocketRequest model to accept a subprotocols: Seq[String] field, but that would break compatibility with current client code. If it, nevertheless, makes sense to do this change, please let us know - we are happy to submit a corresponding PR.

References #2606

[lightbend-cla-validator]

Hi @RomanBrodetski,

Thank you for your contribution! We really value the time you've taken to put this together.

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http://www.lightbend.com/contribute/cla

[akka-ci]

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[raboof]

OK TO TEST

[akka-ci]

Test FAILed.

[raboof]

Looks useful!

[raboof]

(failure was unrelated, #2349)

[raboof]

PLS BUILD

[akka-ci]

Test PASSed.

[jrudolph]

The clean way would be to change WebSocketRequest to support a list of subprotocols directly, however, that might be hard to achieve binary-compatibly.

Could you at least add another apply method to WebSocketRequest that takes a subprotocols: Seq[String] instead of an subprotocol: Option[String], so that this can become the official way of specifying multiple subprotocols?

[akka-ci]

Test PASSed.

[jrudolph]

Thanks @RomanBrodetski. LGTM