- merge() may actually work now, though we've heard that before...

- merge() may actually work now, though we've heard that before...
- merge() uses the priamry key attributes on the object if _instance_key not present.  so merging works for instances that dont have an instnace_key, will still issue UPDATE for existing rows.
- improved collection behavior for merge() - will remove elements from a destination collection that are not in the source.
- fixed naive set-mutation issue in Select._get_display_froms
- simplified fixtures.Base a bit