XML Deserialization fails on relationships when using a mapping file

When XMLDecoder deserializes objects using a mapping descriptor, no objects are registered with a datacontext until after all objects have been initialized. However, cayenne requires a non-null datacontext to correctly handle relationships. Hence, dataobjects using the default cayenne handling of relationships (either to-one or to-many) will be unable to deserialize objects from XML (due to null pointers).

In the case of a collection, the getXXX method will return null, and the XMLMappingDescriptor fails to "sense" a collection.
In the case of a to-one relationship, a NPE exception is thrown trying to set the child property of the parent object.

The simplest way to fix this would be to make XMLMappingDescriptor DataContext-capable (ie: will register objects with a datacontext, if one is provided, as the objects are created). I've done this in a local copy of the code. I will attach a patch when I get a minute.