code: func (tr *transport) RoundTrip(req *http.Request) (*http.Response, error) {
  	tr.initOnce.Do(func() {
  		tr.initErr = tr.initialize()
  	})
  	if tr.initErr != nil {
  		return nil, tr.initErr
  	}
  
  	tlsConfig := &tls.Config{
  		RootCAs:      tr.caCertPool,
  		Certificates: tr.certs,
  	}
  
  	if req.URL.Scheme == "https+insecure" {
  		// Make shallow copy of request, and req.URL, so the request's URL can be
  		// modified.
  		r := *req
  		*r.URL = *req.URL
  		req = &r
  		tlsConfig.InsecureSkipVerify = true
  		req.URL.Scheme = "https"
  	}
  
  	transport := http.Transport{
  		Proxy:           http.ProxyFromEnvironment,
  		TLSClientConfig: tlsConfig,
  	}
  
  	return transport.RoundTrip(req)
  }
docs: // RoundTrip executes a single HTTP transaction, returning
  // a Response for the provided Request.