code: func (js *Js) getTicketFromServer() (ticket resTicket, err error) {
  	var accessToken string
  	accessToken, err = js.GetAccessToken()
  	if err != nil {
  		return
  	}
  
  	var response []byte
  	url := fmt.Sprintf(getTicketURL, accessToken)
  	response, err = util.HTTPGet(url)
  	err = json.Unmarshal(response, &ticket)
  	if err != nil {
  		return
  	}
  	if ticket.ErrCode != 0 {
  		err = fmt.Errorf("getTicket Error : errcode=%d , errmsg=%s", ticket.ErrCode, ticket.ErrMsg)
  		return
  	}
  
  	jsAPITicketCacheKey := fmt.Sprintf("jsapi_ticket_%s", js.AppID)
  	expires := ticket.ExpiresIn - 1500
  	err = js.Cache.Set(jsAPITicketCacheKey, ticket.Ticket, time.Duration(expires)*time.Second)
  	return
  }
docs: //getTicketFromServer 强制从服务器中获取ticket