- contains_eager() now works with the automatically

- contains_eager() now works with the automatically
generated subquery that results when you say
"query(Parent).join(Parent.somejoinedsubclass)", i.e.
when Parent joins to a joined-table-inheritance subclass.
Previously contains_eager() would erroneously add the
subclass table to the query separately producing a
cartesian product.  An example is in the ticket
description.  [ticket:1543]