code: func (msg *Message) SetBase64Payload(payload string) error {
  	str, err := base64.StdEncoding.DecodeString(payload)
  	if err != nil {
  		return fmt.Errorf("could not decode supplied payload using base64: %s", err)
  	}
  
  	msg.Payload = string(str)
  
  	return nil
  }
docs: // SetBase64Payload sets the payload for the message, use it if the payload is Base64 encoded