code: func writeString1(w *bytes.Buffer, s string) (err error) {
  	length := len(s)
  	if length > 255 {
  		return fmt.Errorf("[writeString1] length>255")
  	}
  	w.WriteByte(byte(length))
  	w.WriteString(s)
  	return
  }
docs: //一个字节长度的字符串