refactor(groupBy): clear groups via map.clear interface
refactor(groupBy): clear groups via map.clear interface

closes #1257
ReactiveX/RxJS,kwonoj/RxJS,SangKa/RxJS-Docs-CN,jinroh/RxJS,gluck/RxJS,elliotschi/rxjs,RxJS-CN/RxJS-Docs-CN,gluck/RxJS,ReactiveX/RxJS,adrianomelo/RxJSNext,kwonoj/RxJS,trxcllnt/RxJS,elliotschi/rxjs,benlesh/RxJS,saneyuki/RxJS,benlesh/RxJS,martinsik/rxjs,jinroh/RxJS,benlesh/RxJS,martinsik/rxjs,martinsik/rxjs,Igorbek/rxjs5,trxcllnt/RxJS,SangKa/RxJS-Docs-CN,gluck/RxJS,jinroh/RxJS,Igorbek/rxjs5,adrianomelo/RxJSNext,adrianomelo/RxJS-1,benlesh/RxJS,jayphelps/rxjs,jayphelps/rxjs,jayphelps/rxjs,ReactiveX/rxjs,adrianomelo/RxJS-1,jinroh/RxJS,Brocco/RxJS,figueredo/RxJS,Igorbek/rxjs5,SangKa/RxJS-Docs-CN,figueredo/RxJS,jayphelps/rxjs,adrianomelo/RxJS-1,robwormald/RxJS,Brocco/RxJS,gluck/RxJS,jayphelps/rxjs,staltz/RxJSNext,IgorMinar/RxJS,figueredo/RxJS,robwormald/RxJS,staltz/RxJSNext,RxJS-CN/RxJS-Docs-CN,adrianomelo/RxJSNext,saneyuki/RxJS,adrianomelo/RxJS-1,kwonoj/RxJS,elliotschi/rxjs,ReactiveX/RxJS,RxJS-CN/RxJS-Docs-CN,Igorbek/rxjs5,ReactiveX/rxjs,trxcllnt/RxJS,robwormald/RxJS,kwonoj/RxJS,Brocco/RxJS,SangKa/RxJS-Docs-CN,staltz/RxJSNext,RxJS-CN/RxJS-Docs-CN,adrianomelo/RxJSNext,martinsik/rxjs,IgorMinar/RxJS,ReactiveX/rxjs,ReactiveX/rxjs,Brocco/RxJS,ReactiveX/RxJS,robwormald/RxJS,figueredo/RxJS,elliotschi/rxjs,trxcllnt/RxJS,staltz/RxJSNext