fix System.NotSupportedException

System.NotSupportedException occurred
Message=Cannot convert the value in attribute 'Property' to object of
type 'System.Windows.DependencyProperty'.
Source=PresentationFramework
StackTrace:
at
System.Windows.Markup.DependencyPropertyConverter.ResolveProperty(IServiceProvider
serviceProvider, String targetName, Object source)
Merge pull request #648 from MahApps/647-punker76-MetroDataGridColumnHeader

fix #647 System.NotSupportedException at MetroDataGridColumnHeader style