- fix to "row switch" behavior, i.e. when an INSERT/DELETE is combined into a

- fix to "row switch" behavior, i.e. when an INSERT/DELETE is combined into a
  single UPDATE; many-to-many relations on the parent object update properly.
  [ticket:841]
- it's an error to session.save() an object which is already persistent
  [ticket:840]
- changed a bunch of repr(obj) calls in session.py exceptions to use mapperutil.instance_str()