refactor(toPromise): update type definition for toPromise

- make Promistctor parameter as optional in type definition

closes ReactiveX#864

Author: kwonoj

src/Observable.ts

@@ -244,7 +244,7 @@ export class Observable<T> implements CoreOperators<T>  {
   timeout: (due: number | Date, errorToSend?: any, scheduler?: Scheduler) => Observable<T>;
   timeoutWith: <R>(due: number | Date, withObservable: Observable<R>, scheduler?: Scheduler) => Observable<T> | Observable<R>;
   toArray: () => Observable<T[]>;
-  toPromise: (PromiseCtor: PromiseConstructor) => Promise<T>;
+  toPromise: (PromiseCtor?: PromiseConstructor) => Promise<T>;
   window: (closingNotifier: Observable<any>) => Observable<Observable<T>>;
   windowCount: (windowSize: number, startWindowEvery: number) => Observable<Observable<T>>;
   windowTime: (windowTimeSpan: number, windowCreationInterval?: number, scheduler?: Scheduler) => Observable<Observable<T>>;