ApiReview: Move WebSocket APIs to HttpContext.WebSockets

[Tratcher]

WebSocketRequestedProtocols { get; }
AcceptWebSocketAsync()
AcceptWebSocketAsync(string subProtocol)

But not IsWebSocketRequest { get; }

[Tratcher]

I think it makes sense to also move IsWebSocketRequest in with everything else. Just accessing HttContext.WebSockets doesn't imply any operations. Compare to HttpContext.Authentication.

[davidfowl]

context.WebSockets.IsWebSocketRequest? /cc @lodejard

[Tratcher]

Also do with #273 for IWebSocketAcceptContext.