OffsetX and OffsetY set to zero are not respected

[punker76]

Originally reported by @nicll at punker76#132

I'm creating a ChildWindow using the following code:

ChildWindowManager.ShowChildWindowAsync(Application.Current.MainWindow, dialogView, ChildWindowManager.OverlayFillBehavior.FullWindow);

Here, dialogView is defined in the following way:

<simpleChildWindow:ChildWindow x:Class="Test.ChildView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:simpleChildWindow="clr-namespace:MahApps.Metro.SimpleChildWindow;assembly=MahApps.Metro.SimpleChildWindow"
             ChildWindowWidth="{Binding Width, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Window}}"
             ChildWindowHeight="{Binding Height, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Window}}"
             OffsetX="0"
             OffsetY="0"
             ShowTitleBar="False">
...
</simpleChildWindow:ChildWindow>

As expected, the height and width of the ChildWindow are being set correctly to the same values as the main window.
However, the OffsetX and OffsetY values are being overriden after initialization of the view, causing the ChildWindow to not fill the entire main window.
I assume this is caused by the logic in the ProcessMove method of the ChildWindow.cs class (https://github.com/punker76/MahApps.Metro.SimpleChildWindow/blob/develop/src/MahApps.Metro.SimpleChildWindow/ChildWindow.cs#L1139)

I am using version 2.2.1