code: func redirectPath(path string, r *http.Request) http.Handler {
  	t1 := strings.HasSuffix(path, "/")
  	t2 := strings.HasSuffix(r.URL.Path, "/")
  	if t1 != t2 {
  		u, _ := url.Parse(r.URL.String())
  		if t1 {
  			u.Path += "/"
  		} else {
  			u.Path = u.Path[:len(u.Path)-1]
  		}
  		return http.RedirectHandler(u.String(), 301)
  	}
  	return nil
  }
docs: // redirectPath returns a handler that redirects if the path trailing slash
  // differs from the request URL path.