code: func FileResponder(filename string) func() *http.Response {
  	return func() *http.Response {
  		f, err := os.Open(filename)
  		if err != nil {
  			return &http.Response{StatusCode: 404, Status: "404 Not Found", Body: types.EmptyBody}
  		}
  		return &http.Response{StatusCode: 200, Status: "200 OK", Body: f}
  	}
  }
docs: // FileResponder returns an HTTP response generator that returns the
  // contents of the named file.