title: ReactiveUI 4.1 CreateDerivedCollection(...) filter does not seem to work
body: <p>I am using <code>ReactiveUI 4.1</code>. I use a <code>ReactiveCollection</code> of selectable items (having <code>IsSelected</code> flag) as a source for another derived reactive collection that uses filter to view only items that have <code>IsSelected == true</code>. If the source collection is pre-populated with some un/selected items before creating the derived collection, the view filter seems to work, but when later on items in the source collection go from selected to unselected state these items do not "disappear" within the derived collection. I do set <code>ChangeTrackingEnabled</code> flag to <code>true</code> on all of the collection, but it does not seem to help. Attaching my code snippet: </p>
  
  <pre><code>private readonly ReactiveCollection&lt;string&gt; _sourceItems =  new ReactiveCollection&lt;string&gt;();
  private readonly ReactiveCollection&lt;SelectableDataItem&lt;string&gt;&gt; _selectableItemsView = null;
  private readonly ReactiveCollection&lt;SelectableDataItem&lt;string&gt;&gt; _selectedItemsView = null;
  private readonly ReactiveCollection&lt;string&gt; _selectedDataView = null;
  
  ///....
  
  
      this._sourceItems.ChangeTrackingEnabled = true;
  
      this._selectableItemsView =
          this.SourceItems.CreateDerivedCollection&lt;string, SelectableDataItem&lt;string&gt;&gt;(i =&gt; new SelectableDataItem&lt;string&gt;(i) { IsSelected = true, });
      this._selectableItemsView.ChangeTrackingEnabled = true;
      this._selectedItemsView =
          this._selectableItemsView.CreateDerivedCollection&lt;SelectableDataItem&lt;string&gt;, SelectableDataItem&lt;string&gt;&gt;(
          i =&gt; i,
          f =&gt; f.IsSelected,
          (i1, i2) =&gt; 0
          );
      this._selectedItemsView.ChangeTrackingEnabled = true;
      this._selectedDataView =
          this._selectableItemsView.CreateDerivedCollection&lt;SelectableDataItem&lt;string&gt;, string&gt;(i =&gt; i.Data, f =&gt; f.IsSelected, (i1, i2) =&gt; 0);
      this._selectedDataView.ChangeTrackingEnabled = true;
  </code></pre>