Fix DataGridColumnHeader not using HeaderTemplate

[klotztech]

Hi,
thanks for this awesome library!
I've come to notice, that the DataGrid style overrides the use of my column's HeaderTemplate.

Example

<DataGrid>
    <DataGrid.Columns>
        <DataGridTemplateColumn Header="Header!">
            <DataGridTemplateColumn.HeaderTemplate>
                <DataTemplate>
                    <TextBlock Text="Template!" />
                </DataTemplate>
            </DataGridTemplateColumn.HeaderTemplate>
        </DataGridTemplateColumn>
    </DataGrid.Columns>
</DataGrid>

Actual Output

Expected Output

Proposed Solution

It came down to the ContentTemplate of the DataGridColumnHeader's ContentPresenter not being passed down.
Change required: Use provided HeaderTemplate for DataGridColumnHeader's ContentPresenter

Hope this helps :)
cheers

[Kinnara]

Thanks for contributing!