chore(style): minor modified DataGrid.Small style

src/Shared/HandyControl_Shared/Themes/Styles/Base/CheckBoxBaseStyle.xaml

@@ -19,7 +19,7 @@
         <Setter Property="Background" Value="White"/>
         <Setter Property="BorderBrush" Value="{DynamicResource BorderBrush}"/>
         <Setter Property="BorderThickness" Value="1"/>
-        <Setter Property="HorizontalAlignment" Value="Center"/>
+        <Setter Property="HorizontalAlignment" Value="Stretch"/>
         <Setter Property="VerticalAlignment" Value="Center"/>
         <Setter Property="VerticalContentAlignment" Value="Center"/>
         <Setter Property="Padding" Value="6,0,0,0"/>

src/Shared/HandyControl_Shared/Themes/Styles/Base/DataGridBaseStyle.xaml

@@ -34,7 +34,7 @@
         <Setter Property="BorderBrush" Value="Transparent"/>
         <Setter Property="BorderThickness" Value="0"/>
         <Setter Property="VerticalContentAlignment" Value="Center"/>
-        <Setter Property="HorizontalContentAlignment" Value="Left"/>
+        <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
         <Setter Property="Padding" Value="{StaticResource DefaultControlPadding}"/>
         <Setter Property="hc:BorderElement.CornerRadius" Value="{StaticResource DefaultCornerRadius}"/>
         <Setter Property="Template">