test(filter): add test against breaking unsubscription chain
test(filter): add test against breaking unsubscription chain

Relates to #875.

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