#3070 Stretch the content of the expander header

Microsoft.Toolkit.Uwp.UI.Controls.Layout/Expander/Expander.xaml

@@ -18,7 +18,7 @@
         <Setter Property="Padding" Value="2,0,0,0" />
         <Setter Property="Height" Value="40" />
         <Setter Property="HorizontalAlignment" Value="Stretch" />
-        <Setter Property="HorizontalContentAlignment" Value="Left" />
+        <Setter Property="HorizontalContentAlignment" Value="Stretch" />
         <Setter Property="VerticalAlignment" Value="Stretch" />
         <Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}" />
         <Setter Property="FontWeight" Value="Normal" />