title: setting TabItem.Content with viewmodel is not showing view from data template
body: <p>In code I have a datatemplate in a resource dictionary</p>
  
  <pre><code>&lt;DataTemplate DataType="{x:Type my:ContainerViewModel}"  x:Key="ContainerTemplate"&gt;
      &lt;my:ContainerView/&gt;
  &lt;/DataTemplate&gt;
  </code></pre>
  
  <p>This resource dictionary is added in app.xaml</p>
  
  <pre><code>&lt;ResourceDictionary.MergedDictionaries&gt;
      &lt;ResourceDictionary Source="ResourceDictionaries/DataTemplates.xaml" /&gt;
  &lt;/ResourceDictionary.MergedDictionaries&gt;
  </code></pre>
  
  <p>In the code I am trying to set a tab item with view model</p>
  
  <pre><code>var vm = new ContainerViewModel();
  tabItem.Content = vm;
  </code></pre>
  
  <p>the tabitem just shows the viewmodel name. It is not looking up the data template an showing the view. Any help is appreciated</p>