- query.order_by() now accepts False, which cancels

- query.order_by() now accepts False, which cancels
any existing order_by() state on the Query, allowing
subsequent generative methods to be called which do
not support ORDER BY.  This is not the same as the
already existing feature of passing None, which
suppresses any existing order_by() settings, including
those configured on the mapper.  False will make it
as though order_by() was never called, while
None is an active setting.