fix(fromEvent): make selector argument optional in fromEvent static method
fix(fromEvent): make selector argument optional in fromEvent static method

This makes it consistent with the signature of the implementation.
ReactiveX/rxjs,kwonoj/RxJS,figueredo/RxJS,mattpodwysocki/RxJS,justinwoo/RxJS,ReactiveX/rxjs,benlesh/RxJS,justinwoo/RxJS,jayphelps/rxjs,Igorbek/rxjs5,ReactiveX/rxjs,Igorbek/rxjs5,mattpodwysocki/RxJS,trxcllnt/RxJS,ReactiveX/RxJS,adrianomelo/RxJSNext,staltz/RxJSNext,SangKa/RxJS-Docs-CN,trxcllnt/RxJS,benlesh/RxJS,Brocco/RxJS,ReactiveX/RxJS,staltz/RxJSNext,kwonoj/RxJS,RxJS-CN/RxJS-Docs-CN,jayphelps/rxjs,adrianomelo/RxJS-1,ReactiveX/rxjs,jayphelps/rxjs,robwormald/RxJS,figueredo/RxJS,jinroh/RxJS,kwonoj/RxJS,martinsik/rxjs,figueredo/RxJS,Brocco/RxJS,elliotschi/rxjs,jinroh/RxJS,martinsik/rxjs,staltz/RxJSNext,robwormald/RxJS,Igorbek/rxjs5,martinsik/rxjs,robwormald/RxJS,Brocco/RxJS,jayphelps/rxjs,gluck/RxJS,Igorbek/rxjs5,trxcllnt/RxJS,jadbox/RxJS,adrianomelo/RxJSNext,SangKa/RxJS-Docs-CN,adrianomelo/RxJS-1,jadbox/RxJS,martinsik/rxjs,adrianomelo/RxJS-1,kwonoj/RxJS,justinwoo/RxJS,jadbox/RxJS,Brocco/RxJS,ReactiveX/RxJS,benlesh/RxJS,RxJS-CN/RxJS-Docs-CN,staltz/RxJSNext,jinroh/RxJS,saneyuki/RxJS,IgorMinar/RxJS,gluck/RxJS,mattpodwysocki/RxJS,benlesh/RxJS,mattpodwysocki/RxJS,jayphelps/rxjs,jadbox/RxJS,saneyuki/RxJS,RxJS-CN/RxJS-Docs-CN,robwormald/RxJS,RxJS-CN/RxJS-Docs-CN,elliotschi/rxjs,trxcllnt/RxJS,elliotschi/rxjs,gluck/RxJS,IgorMinar/RxJS,justinwoo/RxJS,SangKa/RxJS-Docs-CN,elliotschi/rxjs,adrianomelo/RxJSNext,SangKa/RxJS-Docs-CN,ReactiveX/RxJS,figueredo/RxJS,jinroh/RxJS,adrianomelo/RxJS-1,adrianomelo/RxJSNext,gluck/RxJS