title: prism - Unable to set binding for HeaderTemplate of TabItem
body: <p>I have a UserControl named ProductEditView which I am adding to a region of my MainWindow (shell). I have a TabControl in Shell region to display the ProductEditView like below</p>
  
  <pre><code>&lt;TabControl Name="tabEditProduct" 
              prism:RegionManager.RegionName="TabContent"
  
      &lt;TabControl.ItemContainerStyle&gt;
      &lt;Style TargetType="TabItem"&gt;
          &lt;Setter Property="HeaderTemplate"&gt;
              &lt;Setter.Value&gt;
                  &lt;DataTemplate&gt;
                      &lt;DockPanel LastChildFill="True" &gt;
                          &lt;Button Width="16" Height="16" Command="{Binding Path=CloseTabCommand}" 
                                          DockPanel.Dock="Right" BorderThickness="0" Padding="0"
                                          BorderBrush="Transparent" Background="Transparent"&gt;
                              &lt;Image Source="Images/close-icon.png" Width="14" Height="14" 
                                             HorizontalAlignment="Stretch" Margin="0"/&gt;
                          &lt;/Button&gt;
                          &lt;TextBlock Text="{Binding ProductName}" 
                                             DockPanel.Dock="Left" Margin="0,0,10,0" /&gt;
                      &lt;/DockPanel&gt;
                  &lt;/DataTemplate&gt;
              &lt;/Setter.Value&gt;
          &lt;/Setter&gt;
      &lt;/Style&gt;
      &lt;/TabControl.ItemContainerStyle&gt;
  &lt;/TabControl&gt;
  </code></pre>
  
  <p>In the TabItem's HeaderTemplate I am trying to display text from the Product Object using binding. Unfortunately the binding is not working, but I can display static text as header. <strong>Is it possible to display the Header Text of Tab Item using binding like above?</strong> </p>
  
  <p>For completeness, I am also pasting the view to Edit Product that loads in the TabControl region as TabItem. The bindings for that are working well. Here is the ProductEditView</p>
  
  <pre><code>&lt;UserControl x:Class="TextExisting.CoreModule.ProductEditView"
  ....
  
          &lt;Grid HorizontalAlignment="Center" DataContext="{Binding Path=Product}" Grid.Row="0" Grid.Column="0"&gt;
          ....
              &lt;TextBlock Text="Id" Grid.Row="0" Grid.Column="0"/&gt;
              &lt;TextBox Grid.Row="0" Grid.Column="1" 
                       Text="{Binding Path=Id, NotifyOnValidationError=True, ValidatesOnExceptions=True}" /&gt;
  
              &lt;TextBlock Text="Product Name" Grid.Row="1" Grid.Column="0"/&gt;
              &lt;TextBox Grid.Row="1" Grid.Column="1" 
                       Text="{Binding Path=ProductName, NotifyOnValidationError=True, ValidatesOnExceptions=True}" /&gt;
              ....
          &lt;/Grid&gt;
          &lt;Button Command="{Binding SaveProductCommand}" Content="Save Product" HorizontalAlignment="Center"/&gt;
  
  &lt;/UserControl&gt;
  </code></pre>