Support Existing Product Class Attributes

First of all, thanks for the great module. It's a big help.

My issue is that I would like to be able to take advantage of the product classes feature of UberCart without having to remove all my attributes from the class before I can run an import.

Currently, I trip up on a primary key constraint when I run my import, because UC Feeds is trying to add the attribute to the product... when the attribute has already been automatically created due to being part of a class.

This is really frustrating, as I really don't want to have to abandon product classes just so that I can use this module to import products.