- Query called in the context of an expression will render

- Query called in the context of an expression will render
disambiguating labels in all cases.    Note that this does
not apply to the existing .statement and .subquery()
accessor/method, which still honors the .with_labels()
setting that defaults to False.

- Query.union() retains disambiguating labels within the
returned statement, thus avoiding various SQL composition
errors which can result from column name conflicts.
[ticket:1676]