code: function genSign(urlQuery, appSecret){
      var urlObj = querystring.parse(urlQuery);
      urlObj.appSecret = appSecret;
  
      var keysSortAsc = Object.keys(urlObj).sort(function(a, b){
          return a.localeCompare(b);
      });
      var valueSortStr = keysSortAsc.map(function(key){
          return urlObj[key];
      }).join('');
  
      var md5 = crypto.createHash('md5');
      return md5.update(valueSortStr, 'utf8').digest('hex');
  }
docs: 签名生成
  
  @param {String} urlQuery `url query 部分字符串`
  @param {String} appSecret `帐号密匙`