[MWF] Fix PropertyGridTest.PropertyGrid_ArrayParentTest by returning the correct value of PropertyCategoryDefault

[This code](https://github.com/mono/referencesource/blob/7dc4e513eafd3fb30d34111948ef14c981264463/System/compmod/system/componentmodel/CategoryAttribute.cs#L276) in ComponentModel looks up the category name by string concatenation, which means it wasn't captured in our SR.cs.
We also can't add the correct value for PropertyCategoryDefault as a string constant to SR.cs since the value is retrieved via GetObject () and we currently just return the parameter name there.

A short-term workaround is to just special case PropertyCategoryDefault in GetObject ().