Major Description
In cases where the collection-descriptor of a field of type List/Set does not specify collection-class, OJB currently uses RemovalAwareCollection which is a List but not a Set.
Instead, OJB should use the field type, and either throw a metadata exception if none of the default collection classes can be assigned to it, or use a fitting default collection.
This also means that a RemovalAwareSet is needed (based upon ManageableHashSet).