test(groupBy): fix misbehaving groupBy test
test(groupBy): fix misbehaving groupBy test

Closes issue #1743.

ReactiveX/RxJS,elliotschi/rxjs,trxcllnt/RxJS,adrianomelo/RxJS-1,gluck/RxJS,ReactiveX/rxjs,ReactiveX/rxjs,ReactiveX/rxjs,adrianomelo/RxJSNext,jayphelps/rxjs,ReactiveX/RxJS,RxJS-CN/RxJS-Docs-CN,ReactiveX/RxJS,trxcllnt/RxJS,jayphelps/rxjs,Brocco/RxJS,staltz/RxJSNext,staltz/RxJSNext,jayphelps/rxjs,jayphelps/rxjs,Brocco/RxJS,martinsik/rxjs,SangKa/RxJS-Docs-CN,kwonoj/RxJS,elliotschi/rxjs,gluck/RxJS,adrianomelo/RxJSNext,adrianomelo/RxJSNext,benlesh/RxJS,adrianomelo/RxJS-1,kwonoj/RxJS,martinsik/rxjs,RxJS-CN/RxJS-Docs-CN,gluck/RxJS,benlesh/RxJS,figueredo/RxJS,trxcllnt/RxJS,gluck/RxJS,staltz/RxJSNext,ReactiveX/RxJS,ReactiveX/rxjs,adrianomelo/RxJS-1,SangKa/RxJS-Docs-CN,jayphelps/rxjs,Brocco/RxJS,IgorMinar/RxJS,martinsik/rxjs,SangKa/RxJS-Docs-CN,SangKa/RxJS-Docs-CN,kwonoj/RxJS,adrianomelo/RxJS-1,RxJS-CN/RxJS-Docs-CN,trxcllnt/RxJS,martinsik/rxjs,elliotschi/rxjs,adrianomelo/RxJSNext,figueredo/RxJS,figueredo/RxJS,RxJS-CN/RxJS-Docs-CN,kwonoj/RxJS,elliotschi/rxjs,Brocco/RxJS,IgorMinar/RxJS,staltz/RxJSNext,benlesh/RxJS,benlesh/RxJS,figueredo/RxJS