docs: function meta_data_still_fresh(item) {
      var promise = read_meta_data(item).
      then(function(item) {
          return new Promise(function(resolve, reject) {
              if (headers_still_fresh(item.meta_data.headers)) {
                  resolve(item);
              } else {
                  reject(item);
              }
          });
      });
  
      return promise;
  }