code: func checkHeaders(headers map[string]string, caller string) map[string]string {
  	if headers == nil {
  		headers = map[string]string{}
  	}
  	// We don't just check to see if there's a value; it has to be non-empty.
  	if headers["X-Uber-Source"] == "" {
  		headers["X-Uber-Source"] = caller
  	}
  	return headers
  }
docs: // checkHeaders ensures that X-Uber-Source is set.