fix: ObserveCountChanged misses count if observer has side effect

[awesomestmason]

The issue is that OnNext can have any number of side effects. If a side effect modifies the collection.Count, then the countPrev will be inaccurate, because the collection.Count has changed. It was not the same value that was actually emitted.

        protected override void Handler(in NotifyCollectionChangedEventArgs<T> eventArgs)
        {
            switch (eventArgs.Action)
            {
                case NotifyCollectionChangedAction.Add:
                case NotifyCollectionChangedAction.Remove:
                case NotifyCollectionChangedAction.Reset when countPrev != collection.Count:
                    observer.OnNext(collection.Count);
                    break;
            }
            countPrev = collection.Count; // Issue: collection.Count be a different value from the emitted value above, if OnNext has side effects.
        }

To fix this, I moved the countPrev assignment to before the emission. That way it is guaranteed that countPrev matches the value that we actually emitted.

[awesomestmason]

closes #120