change the websocket.Upgrade function signature

[bestgopher]

Current the Upgrade function take a http.ResponseWriter as first argument, but if the argument is not implemented the http.Hijacker interface, an error will be raised at running time.
I think the better way is check the argument is valid at compile time.

We can define a new interface that combines the http.ResponseWriter and http.Hijacker. And then change the Upgrade function .

type WebsocketResponseWriter interface {
    http.ResponseWriter
    http.Hijacker
}

func Upgrade(w WebsocketResponseWriter, r *http.Request, responseHeader http.Header) {}