test(withLatestFrom): add unsubscription-related specs
test(withLatestFrom): add unsubscription-related specs

Add test case to verify that unsubscription chains are not broken. Add test that asserts
subscriptions when an unsubscription happens explicitly.

Related to issue #875.

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