fixes for case adding data at the end of the textbox

Author: rizamarhaban

Microsoft.Toolkit.Uwp.UI/Extensions/TextBoxMask/TextBoxMask.cs

@@ -300,7 +300,7 @@ private static void Textbox_TextChanging(TextBox textbox, TextBoxTextChangingEve
             // case adding data at the end of the textbox
             if (oldSelectionStart >= oldText.Length && !isDeleteOrBackspace)
             {
-                textbox.Text = oldText;
+                textbox.Text = textbox.Text.Substring(0, oldText.Length);
                 if (oldText.Length >= 0)
                 {
                     textbox.SelectionStart = oldText.Length;