message: refactor(mergeAll): fix typings
diff: import { mergeMap } from './mergeMap';
  import { identity } from '../util/identity';
  -import { OperatorFunction, ObservableInput } from '../types';
  +import { OperatorFunction, ObservableInput, ObservedValueOf } from '../types';
  /**
  * Converts a higher-order Observable into a first-order Observable which
  @@ -58,6 +58,6 @@ import { OperatorFunction, ObservableInput } from '../types';
  * @return {Observable} An Observable that emits values coming from all the
  * inner Observables emitted by the source Observable.
  */
  -export function mergeAll<T>(concurrent: number = Infinity): OperatorFunction<ObservableInput<T>, T> {
  +export function mergeAll<O extends ObservableInput<any>>(concurrent: number = Infinity): OperatorFunction<O, ObservedValueOf<O>> {
  return mergeMap(identity, concurrent);
  }
  