- the _Label construct, i.e. the one that is produced

- the _Label construct, i.e. the one that is produced
whenever you say somecol.label(), now counts itself
in its "proxy_set" unioned with that of it's
contained column's proxy set, instead of
directly returning that of the contained column.
This allows column correspondence
operations which depend on the identity of the
_Labels themselves to return the correct result
- fixes ORM bug [ticket:1852].