server.go
func ListenAndServe(addr string, handler RequestHandler) error {
	s := &Server{
		Handler: handler,
	}
	return s.ListenAndServe(addr)
}
func ListenAndServe(addr string, handler RequestHandler) error {
	s := &Server{
		Handler: handler,
	}
	return s.ListenAndServe(addr)
}
// ListenAndServe serves HTTP requests from the given TCP addr
// using the given handler.