Fix for issue #511 - AlternatingRowBackground

This fixes https://github.com/MahApps/MahApps.Metro/issues/511. 

AlternatingRowBackground must be set from a DataGrid style to work correctly with this fix.

````c#
<DataGrid ItemsSource="{Binding Path=Albums}" 
		  Grid.Row="0"
		  AutoGenerateColumns="False">
	<DataGrid.Style>
		<Style TargetType="DataGrid" BasedOn="{StaticResource {x:Type DataGrid}}">
			<Setter Property="AlternatingRowBackground" Value="#FF282828"/>
		</Style>
	</DataGrid.Style>
	<DataGrid.Columns>
		<DataGridTextColumn Header="Title" Binding="{Binding Title}" />
		<DataGridTextColumn Header="Artist" Binding="{Binding Artist.Name}" />
		<DataGridTextColumn Header="Genre" Binding="{Binding Genre.Name}" />
		<DataGridTextColumn Header="Price" Binding="{Binding Price,StringFormat=c}" />
	</DataGrid.Columns>
</DataGrid>
````