io/flushing_writer.go
func (w *FlushingWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
	if hijacker, ok := w.ResponseWriter.(http.Hijacker); ok {
		return hijacker.Hijack()
	}
	return nil, nil, errors.New("cannot hijack connection")
}
func (w *FlushingWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
	if hijacker, ok := w.ResponseWriter.(http.Hijacker); ok {
		return hijacker.Hijack()
	}
	return nil, nil, errors.New("cannot hijack connection")
}
// Hijack will hijack the underlying TCP connection, if available in the
// ResponseWriter.