code: function each(collection, callback) {
          if (!collection) {
              return;
          }
  
          for (var i = 0, l = collection.length; i < l; i += 1) {
              callback(collection[i]);
          }
      }
docs: filtering to enable a white-list version of Sinon FakeXhr, where whitelisted requests are passed through to real XHR