code: func (c *Conn) GetSource(index string, _type string, id string, args map[string]interface{}, source interface{}) error {
  	url := fmt.Sprintf("/%s/%s/%s/_source", index, _type, id)
  	body, err := c.DoCommand("GET", url, args, nil)
  	if err == nil {
  		err = json.Unmarshal(body, &source)
  	}
  	return err
  }
docs: // GetSource retrieves the document by id and converts it to provided interface