test(Subscription): add test for adding errant custom subscription to unsubscribed subscription
test(Subscription): add test for adding errant custom subscription to unsubscribed subscription

- tries to synchronously add a custom subscription that throws when unsubscribed to a Subscription that is mid-unsubscribe and therefor
  flagged `isUnsubscribed`. It's an edge-case, but it should work.

fixes #1256

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