docs: public List<UserSummary> getUserSummary(Date start, Date end) {
          if(diffThan6Days(start, end)) {
              throw new IllegalArgumentException("start和end相差不能超过6天以上");
          }
  
          String url = WxEndpoint.get("url.stats.user.summary");
          String json = "{\"begin_date\":\"%s\",\"end_date\":\"%s\"}";
  
          DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
          String body = String.format(json, dateFormat.format(start), dateFormat.format(end));
          logger.debug("stats: get user summary: {}", body);
          String response = wxClient.post(url, body);
          UserSummaryWrapper userSummaryWrapper = JsonMapper.defaultMapper().fromJson(response, UserSummaryWrapper.class);
          return userSummaryWrapper.getList();
      }