code: func WriteString(w http.ResponseWriter, format string, data []interface{}) (err error) {
  	writeContentType(w, plainContentType)
  	if len(data) > 0 {
  		_, err = fmt.Fprintf(w, format, data...)
  		return
  	}
  	_, err = io.WriteString(w, format)
  	return
  }
docs: // WriteString writes data according to its format and write custom ContentType.