fix(groupBy): unsubscribe GroupDurationSubscriber after completing th…

…e group

The Groups are disposed by the GroupDurationSelector, however the
GroupDurationSubscriber can be subscribed to a different observable
than the group itself. To prevent any unwanted subscriptions to
accumulate over time we need to explicitly unsubscribe after the
first event in GroupDurationSubscriber closes the group.

Fixes ReactiveX#2660

src/operator/groupBy.ts

@@ -240,6 +240,7 @@ class GroupDurationSubscriber<K, T> extends Subscriber<T> {
       group.error(err);
     }
     this.parent.removeGroup(this.key);
+    this.unsubscribe();
   }
 
   protected _complete(): void {
@@ -248,6 +249,7 @@ class GroupDurationSubscriber<K, T> extends Subscriber<T> {
       group.complete();
     }
     this.parent.removeGroup(this.key);
+    this.unsubscribe();
   }
 }