message: fix(mergeAll): introduce variant support <T, R> for mergeMap
  closes
diff: @@ -3,8 +3,12 @@ import { Operator } from '../Operator';
  import { Observer } from '../Observer';
  import { Subscription } from '../Subscription';
  import { OuterSubscriber } from '../OuterSubscriber';
  +import { Subscribable } from '../Observable';
  import { subscribeToResult } from '../util/subscribeToResult';
  +export function mergeAll<T>(this: Observable<T>, concurrent?: number): T;
  +export function mergeAll<T, R>(this: Observable<T>, concurrent?: number): Subscribable<R>;
  +
  /**
  * Converts a higher-order Observable into a first-order Observable which
  * concurrently delivers all values that are emitted on the inner Observables.
  