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 #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();
  }
 }