code: func (ctx *Context) GetAccessTokenFromServer() (resAccessToken ResAccessToken, err error) {
  	url := fmt.Sprintf("%s?grant_type=client_credential&appid=%s&secret=%s", AccessTokenURL, ctx.AppID, ctx.AppSecret)
  	var body []byte
  	body, err = util.HTTPGet(url)
  	if err != nil {
  		return
  	}
  	err = json.Unmarshal(body, &resAccessToken)
  	if err != nil {
  		return
  	}
  	if resAccessToken.ErrMsg != "" {
  		err = fmt.Errorf("get access_token error : errcode=%v , errormsg=%v", resAccessToken.ErrCode, resAccessToken.ErrMsg)
  		return
  	}
  
  	accessTokenCacheKey := fmt.Sprintf("access_token_%s", ctx.AppID)
  	expires := resAccessToken.ExpiresIn - 1500
  	err = ctx.Cache.Set(accessTokenCacheKey, resAccessToken.AccessToken, time.Duration(expires)*time.Second)
  	return
  }
docs: //GetAccessTokenFromServer 强制从微信服务器获取token