Dial creates a new client connection. Use requestHeader to specify the origin (Origin), subprotocols (Sec-WebSocket-Protocol) and cookies (Cookie). Use the response.Header to get the selected subprotocol (Sec-WebSocket-Protocol) and cookies (Set-Cookie).
If the WebSocket handshake fails, ErrBadHandshake is returned along with a non-nil *http.Response so that callers can handle redirects, authentication, etcetera. The response body may not contain the entire response and does not need to be closed by the application.