code: func (st V4Transport) RoundTrip(req *http.Request) (*http.Response, error) {
  	// Instead of directly modifying the request then calling http.DefaultTransport,
  	// instead restart the request with the HTTPClient.Do function,
  	// because the HTTPClient includes safeguards around not forwarding the
  	// signed Authorization header to untrusted domains.
  	return st.HTTPClient.Do(awsauth.Sign4(req, st.Credentials))
  }
docs: // RoundTrip uses the underlying RoundTripper transport, but signs request first with AWS V4 Signing