fix(httpBackend): should not read statusText when request is aborted

Commit https://github.com/angular/angular.js/commit/1d2414ca93a0340840ea1e80c48edb51ec55cd48 introduced a regression which breaks IE9 by accessing the `statusText` property of an aborted xhr request. There's been a similar problem earlier which was fixed with commit https://github.com/gonzaloruizdevilla/angular.js/commit/c882b26062d4030f48d7c50d3e469669b910ea9f.

This PR changes the behavior to only access the `statusText` property if the request wasn't aborted.