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

Add test for share() operator, to verify that unsubscription chains are not broken when the last
subscriber unsubscribes.

For issue #875.

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