Added failing test for #1548.

Caused by a bug in Portable.Xaml: https://github.com/cwensley/Portable.Xaml/issues/95

tests/Avalonia.Markup.Xaml.UnitTests/Xaml/BasicTests.cs

@@ -81,6 +81,21 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
             Assert.Equal(21.0, TextBlock.GetFontSize(target));
         }
 
+        [Fact]
+        public void Attached_Property_Is_Set_On_Control_Outside_Avalonia_Namspace()
+        {
+            // Test for issue #1548
+            var xaml =
+@"<UserControl xmlns='https://github.com/avaloniaui'
+    xmlns:local='clr-namespace:Avalonia.Markup.Xaml.UnitTests.Xaml;assembly=Avalonia.Markup.Xaml.UnitTests'>
+  <local:TestControl Grid.Column='2' />
+</UserControl>";
+
+            var target = AvaloniaXamlLoader.Parse<UserControl>(xaml);
+
+            Assert.Equal(2, Grid.GetColumn((TestControl)target.Content));
+        }
+
         [Fact]
         public void Attached_Property_With_Namespace_Is_Set()
         {