'config.add_propertysheet' conflicts silently when passed class instead of interface

[tseaver]

I defined three content types, each deriving from substanced.file.File (see https://gist.github.com/tseaver/4857cac66e274c9b3117). When adding them using custom add views, the correct schemas are displayed. However, for both the Bill and Contract types, the Invoice propertysheet is found / displayed.

I can work around the problem by assigning unique interfaces to each content type, and calling add_propertysheet with the interface, rather than the content type class.