docs: public AccessToken getAccessToken(String code) {
          String url = WxEndpoint.get("url.oauth.accesstoken.get");
          String formatUrl = String.format(url, wxClient.getClientId(), wxClient.getClientSecret(), code);
          logger.debug("get access token: {}", formatUrl);
          String response = wxClient.get(formatUrl, false);
          return JsonMapper.defaultMapper().fromJson(response, AccessToken.class);
      }