code: func Send(client *http.Client, sub *Subscription, message, token string) (*http.Response, error) {
  	if client == nil {
  		client = http.DefaultClient
  	}
  
  	req, err := NewPushRequest(sub, message, token)
  	if err != nil {
  		return nil, err
  	}
  
  	return client.Do(req)
  }
docs: // Send a message using the Web Push protocol to the recipient identified by the
  // given subscription object. If the client is nil then the default HTTP client
  // will be used. If the push service requires an authentication header (notably
  // Google Cloud Messaging, used by Chrome) then you can add that as the token
  // parameter.