Make session.merge respect entity_name

[Former user]

As discussed on IRC, this patch makes session.merge respect entity_name.

[Mike Bayer]

OK, not exactly, the correct patch is in changeset:2505. this propigates entity_name for transient instances only, meaning if the given object is already session.save()d, it will use the entity_name thats already on the instance.

so to save a new instance to two mappers, dont use session.save() at all. use:

  instance = MyObject(<values>)
  session.merge(instance, entity_name=None)
  session.merge(instance, entity_name='foo')
  session.merge(instance, entity_name='bar')
  session.flush()

[Mike Bayer]

continuing on #543