if overriding tax_category, it only makes sense to update tax_categor…

…y_id as well

fixes confusing display of Tax category as "None" in admin, while it actually uses "Default"
ideally, it would be better to update the tax_category_id in the DB and re-update if
TaxCategory#is_default changes

core/app/models/spree/product.rb

@@ -220,6 +220,10 @@ def tax_category
       @tax_category ||= super || TaxCategory.find_by(is_default: true)
     end
 
+    def tax_category_id
+      super || tax_category.try(:id)
+    end
+
     # Adding properties and option types on creation based on a chosen prototype
     attr_accessor :prototype_id