fix(mergeAll): use higher-order lettable version of mergeAll
fix(mergeAll): use higher-order lettable version of mergeAll

trxcllnt/RxJS,kwonoj/RxJS,trxcllnt/RxJS,trxcllnt/RxJS,jayphelps/rxjs,jayphelps/rxjs,ReactiveX/RxJS,ReactiveX/RxJS,kwonoj/RxJS,IgorMinar/RxJS,martinsik/rxjs,kwonoj/RxJS,ReactiveX/rxjs,jayphelps/rxjs,benlesh/RxJS,IgorMinar/RxJS,Brocco/RxJS,ReactiveX/rxjs,jayphelps/rxjs,benlesh/RxJS,ReactiveX/rxjs,Brocco/RxJS,ReactiveX/RxJS,Brocco/RxJS,benlesh/RxJS,ReactiveX/RxJS,martinsik/rxjs,kwonoj/RxJS,benlesh/RxJS,ReactiveX/rxjs,martinsik/rxjs,martinsik/rxjs,Brocco/RxJS,jayphelps/rxjs,trxcllnt/RxJS