Slightly improve the disabled attribute test coverage
Slightly improve the disabled attribute test coverage

maxamillion32/ngInfiniteScroll,AmitThakkar/ngInfiniteScroll,Masadow/ngInfiniteScroll,just-boris/ngInfiniteScroll,timesqueezer/ngInfiniteScroll,suryasingh/ngInfiniteScroll,aisharagheb/ngInfiniteScroll,msbit/ngInfiniteScroll,xuwupeng2000/ngInfiniteScroll,itkin/ngInfiniteScroll,tianyawy/ngInfiniteScroll,sroze/ngInfiniteScroll,sroze/ngInfiniteScroll,uzen/angular-infinitescroll,cloudnode-app/ngInfiniteScroll,Tradiio/ngInfiniteScroll,maksimr/ngInfiniteScroll,seawenzhu/ngInfiniteScroll,yesmeck/ngInfiniteScroll,Manumental32/ngInfiniteScroll,solomon87/ngInfiniteScroll,uzen/angular-infinitescroll