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

* test(debounce): add failing scalar selector test

* fix(debounce): support scalar selectors

Closes #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);
     }
   }