code: func writeString2(w *bytes.Buffer, s string) (err error) {
  	length := len(s)
  	if length > 65535 {
  		return fmt.Errorf("[writeString2] length>65535")
  	}
  	binary.Write(w, binary.BigEndian, uint16(length))
  	w.WriteString(s)
  	return
  }
docs: //两个字节长度的字符串