function done(status, response, headersString, statusText, xhrStatus) {
        if (cache) {
          if (isSuccess(status)) {
            cache.put(url, [status, response, parseHeaders(headersString), statusText, xhrStatus]);
          } else {
            // remove promise from the cache
            cache.remove(url);
          }
        }

        function resolveHttpPromise() {
          resolvePromise(response, status, headersString, statusText, xhrStatus);
        }

        if (useApplyAsync) {
          $rootScope.$applyAsync(resolveHttpPromise);
        } else {
          resolveHttpPromise();
          if (!$rootScope.$$phase) $rootScope.$apply();
        }
      }
Callback registered to $httpBackend():
 - caches the response if desired
 - resolves the raw $http promise
 - calls $apply