fix(throttle): ChildSubscriptions removed on inner unsubscribe (React…

…iveX#2355)

Hold reference to ChildSubscription, not Subscription, so it can be properly removed later.

src/operator/throttle.ts

@@ -89,14 +89,13 @@ class ThrottleSubscriber<T, R> extends OuterSubscriber<T, R> {
  }
 
  private emitAndThrottle(value: T, duration: SubscribableOrPromise<number>) {
- this.add(this.throttled = subscribeToResult(this, duration));
+ this.throttled = this.add(subscribeToResult(this, duration));
  this.destination.next(value);
  }
 
  protected _unsubscribe() {
  const throttled = this.throttled;
  if (throttled) {
- this.remove(throttled);
  this.throttled = null;
  throttled.unsubscribe();
  }