title: MultiBinding with MultiValueConverter does not update
body: <p>it seems that I have a problem with my multibinding.</p>
  
  <p>Scenario:<br>
  I have a window with two datepickers and a listview.
  The listliew contains some data bound elements called "entries". An entry has a property called "date".</p>
  
  <p>I just want my listview to show entries whose date is in between my two datepickes dates.</p>
  
  <p>My xaml code for binding the listview to the entries and dates:</p>
  
  <pre><code>&lt;ListView.ItemsSource&gt;
    &lt;MultiBinding Converter="{StaticResource EntriesFilterConv}"
                  UpdateSourceTrigger="PropertyChanged"&gt;
      &lt;Binding Path="Entries" UpdateSourceTrigger="PropertyChanged"/&gt;
      &lt;Binding ElementName="EntryFromDate" Path="SelectedDate"
               UpdateSourceTrigger="PropertyChanged"/&gt;
      &lt;Binding ElementName="EntryToDate" Path="SelectedDate"
               UpdateSourceTrigger="PropertyChanged"/&gt;
    &lt;/MultiBinding&gt;
  &lt;/ListView.ItemsSource&gt;
  </code></pre>
  
  <p>However, this doesnt work. My converter is called when a SelectedDate changes but its never called when Entries changes.</p>
  
  <p>With normal data binding like this:</p>
  
  <pre><code>&lt;ListView ItemsSource="{Binding Entries}"&gt;
    ...
  &lt;/ListView&gt;
  </code></pre>
  
  <p>The listview updates normally.
  Any idea?</p>