Fix the issue that GroupBy may not call 'unsubscribe'

> We shouldn't call unsubscribe before emitting onCompleted.

If it becomes 0, it means an asynchronous `unsubscribe` has already been called from:
```Java
            child.add(Subscriptions.create(new Action0() {

                @Override
                public void call() {
                    if (WIP_FOR_UNSUBSCRIBE_UPDATER.decrementAndGet(self) == 0) {
                        self.unsubscribe();
                    }
                }

            }));
```
So now `child.isUnsubscribed` must be `true` and `onCompleted` won't be called.