Fix Thumb of GridViewColumnHeader.

Author: lyrise

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.ListView.xaml

@@ -89,6 +89,7 @@
                     <DockPanel>
                         <Thumb x:Name="PART_HeaderGripper"
                                DockPanel.Dock="Right"
+                               Margin="0,0,-8,0"
                                Style="{StaticResource MaterialDesignGridColumnHeaderGripper}" />
                         <Border x:Name="HeaderBorder"
                                 Padding="{TemplateBinding Padding}"