code: func (wxa *MiniProgram) fetchCode(urlStr string, body interface{}) (response []byte, err error) {
  	var accessToken string
  	accessToken, err = wxa.GetAccessToken()
  	if err != nil {
  		return
  	}
  
  	urlStr = fmt.Sprintf(urlStr, accessToken)
  	var contentType string
  	response, contentType, err = util.PostJSONWithRespContentType(urlStr, body)
  	if err != nil {
  		return
  	}
  	if strings.HasPrefix(contentType, "application/json") {
  		// 返回错误信息
  		var result util.CommonError
  		err = json.Unmarshal(response, &result)
  		if err == nil && result.ErrCode != 0 {
  			err = fmt.Errorf("fetchCode error : errcode=%v , errmsg=%v", result.ErrCode, result.ErrMsg)
  			return nil, err
  		}
  	} else if contentType == "image/jpeg" {
  		// 返回文件
  		return response, nil
  	} else {
  		err = fmt.Errorf("fetchCode error : unknown response content type - %v", contentType)
  		return nil, err
  	}
  
  	return
  }
docs: // fetchCode 请求并返回二维码二进制数据