Websocket Subprotocol support

[barthorre]

It seems there is no support for adding subprotocols to a websocket request.

For Akka backend you can customize the websocket request directly to solve the issue:

AkkaHttpBackend(customizeWebsocketRequest = { request => request.copy(subprotocol = Some("protocol)) })

Others allow you to manually set the Sec-WebSocket-Protocol header directly. (AHC for instance)
HttpClient however throws an exception when trying to add the header.

Are there plans to support subprotocols in a generic way across all backends?
Or can I just open a PR adding it to HttpClientAsyncBackend, since that is what I need at the moment?

[adamw]

HttpClient support is done - by handling the headers appropriately.

Would be great to implement this in other backends as well, & add a test :)