title: CompositeCollection doesn't propagate changes in underlying collections
body: <p>I have what I thought was an ideal scenario for a CompositeCollection, except it seems that changes <strong>to the items of the</strong> underlying collection do not appear in the CompositeCollection (or, at any rate, not in the control whose source is this CompositeCollection).</p>
  
  <p><strong>EDIT 1</strong>: both underlying collections are ObservableCollections.<br>
  <strong>EDIT 2</strong>: the new/updated item gets added, but the contents of that item are not reflected in the drop-down area of the combobox. Each item implements INotifyPropertyChanged.</p>
  
  <p>Am I doing something wrong or is this not supported?</p>
  
  <p>Here's what I have:</p>
  
  <pre><code>&lt;ComboBox SelectedItem="{Binding Products}"&gt;
     &lt;ComboBox.Resources&gt;
        &lt;CollectionViewSource x:Key="CustomProductsSource" Source="{Binding CustomProducts}" /&gt;
     &lt;/ComboBox.Resources&gt;
     &lt;ComboBox.ItemsSource&gt;
        &lt;CompositeCollection&gt;
            &lt;CollectionContainer Collection="{Binding Source={x:Static local:Products.Standard}}" /&gt;
            &lt;Separator/&gt;
            &lt;CollectionContainer Collection="{Binding Source={StaticResource CustomProductsSource}}"/&gt;
        &lt;/CompositeCollection&gt;                                        
      &lt;/ComboBox.ItemsSource&gt;                                     
  &lt;/ComboBox&gt;
  </code></pre>