fix(debounce): support scalar selectors (ReactiveX#3236)

* test(debounce): add failing scalar selector test

* fix(debounce): support scalar selectors

Closes ReactiveX#3232

* test(debounce): rename and comment test

spec/operators/debounce-spec.ts

@@ -44,6 +44,19 @@ describe('Observable.prototype.debounce', () => {
  expectSubscriptions(e1.subscriptions).toBe(e1subs);
  });
 
+ it('should support a scalar selector observable', () => {
+
+ // If the selector returns a scalar observable, the debounce operator
+ // should emit the value immediately.
+
+ const e1 = hot('--a--bc--d----|');
+ const e1subs = '^ !';
+ const expected = '--a--bc--d----|';
+
+ expectObservable(e1.debounce(() => Rx.Observable.of(0))).toBe(expected);
+ expectSubscriptions(e1.subscriptions).toBe(e1subs);
+ });
+
  it('should complete when source does not emit', () => {
  const e1 = hot('-----|');
  const e1subs = '^ !';

src/internal/operators/debounce.ts

@@ -105,7 +105,7 @@ class DebounceSubscriber<T, R> extends OuterSubscriber<T, R> {
  }
 
  subscription = subscribeToResult(this, duration);
- if (!subscription.closed) {
+ if (subscription && !subscription.closed) {
  this.add(this.durationSubscription = subscription);
  }
  }