Fix regression introduced with https://github.com/angular/angular.js/com...

Fix regression introduced with https://github.com/angular/angular.js/commit/7b6c1d08aceba6704a40302f373400aed9ed0e0b breaking e.g. module https://github.com/nervgh/angular-file-upload: Be more robust about missing header object. https://github.com/angular/angular.js/issues/2973#issuecomment-59181169