more XMLMappingDescriptor woes. :)

[Robert Zeigler]

XMLMappingDescriptor chokes on decoding collections for  
CayenneDataObjects.
Cause:
	1) CayenneDataObject.readProperty returns null (instead of an empty  
list) for collection properties when the ObjectContext of the  
CayenneDataObject is null
	2) XMLMappingDescriptor is using an instanceof operator to determine  
whether a property is a collection or not.

I consider this more of a problem with the readProperty method of  
CayenneDataObject than an issue with XMLMappingDescriptor.  It would  
be nice if readProperty returned an empty list for to-many  
relationships, even when there's no data/object context.

Thoughts?

For now, I've skirted the issue by having all of my data objects  
extend a custom superclass, overriding the readProperty, and using a  
bit of reflection to return an empty ToManyList when the returned  
value is supposed to be a collection but the ObjectContext is null.  
It seems to work... but I'm not sure of the repercussions of doing  
this...?

Robert