test(distinctUntilKeyChanged): add unsubscription-related tests
test(distinctUntilKeyChanged): add unsubscription-related tests

Add tests for distinctUntilKeyChanged operator, to verify that it behaves correctly when the result
Observable is unsubscribed.

gluck/RxJS,kwonoj/RxJS,martinsik/rxjs,adrianomelo/RxJSNext,adrianomelo/RxJS-1,gluck/RxJS,jayphelps/rxjs,Igorbek/rxjs5,benlesh/RxJS,jadbox/RxJS,figueredo/RxJS,elliotschi/rxjs,robwormald/RxJS,ReactiveX/RxJS,benlesh/RxJS,Brocco/RxJS,ReactiveX/rxjs,kwonoj/RxJS,gluck/RxJS,Igorbek/rxjs5,jayphelps/rxjs,Igorbek/rxjs5,jinroh/RxJS,jadbox/RxJS,figueredo/RxJS,jayphelps/rxjs,SangKa/RxJS-Docs-CN,justinwoo/RxJS,benlesh/RxJS,ReactiveX/rxjs,elliotschi/rxjs,staltz/RxJSNext,justinwoo/RxJS,martinsik/rxjs,kwonoj/RxJS,IgorMinar/RxJS,jinroh/RxJS,trxcllnt/RxJS,jadbox/RxJS,elliotschi/rxjs,saneyuki/RxJS,martinsik/rxjs,SangKa/RxJS-Docs-CN,jinroh/RxJS,RxJS-CN/RxJS-Docs-CN,RxJS-CN/RxJS-Docs-CN,trxcllnt/RxJS,trxcllnt/RxJS,ReactiveX/RxJS,figueredo/RxJS,staltz/RxJSNext,justinwoo/RxJS,robwormald/RxJS,robwormald/RxJS,adrianomelo/RxJS-1,jayphelps/rxjs,ReactiveX/RxJS,gluck/RxJS,jinroh/RxJS,RxJS-CN/RxJS-Docs-CN,adrianomelo/RxJSNext,adrianomelo/RxJS-1,jayphelps/rxjs,SangKa/RxJS-Docs-CN,adrianomelo/RxJSNext,elliotschi/rxjs,ReactiveX/rxjs,kwonoj/RxJS,jadbox/RxJS,IgorMinar/RxJS,Brocco/RxJS,staltz/RxJSNext,robwormald/RxJS,Brocco/RxJS,ReactiveX/RxJS,RxJS-CN/RxJS-Docs-CN,adrianomelo/RxJS-1,staltz/RxJSNext,Brocco/RxJS,benlesh/RxJS,trxcllnt/RxJS,martinsik/rxjs,Igorbek/rxjs5,ReactiveX/rxjs,saneyuki/RxJS,figueredo/RxJS,SangKa/RxJS-Docs-CN,justinwoo/RxJS,adrianomelo/RxJSNext