fix cost_object validation

- would not work for invalid ids
- use "inclusion" message instead of generic "invalid" message

lib/open_project/costs/patches/work_package_patch.rb

@@ -99,9 +99,9 @@ module OpenProject::Costs::Patches::WorkPackagePatch
     end
 
     def validate_cost_object
-      if cost_object && cost_object.changed?
-        unless (cost_object.blank? || project.cost_object_ids.include?(cost_object.id))
-          errors.add :cost_object, :invalid
+      if changed.include? 'cost_object_id'
+        unless (cost_object_id.blank? || project.cost_object_ids.include?(cost_object_id))
+          errors.add :cost_object, :inclusion
         end
       end
     end