- Query.with_polymorphic() now accepts a third

- Query.with_polymorphic() now accepts a third
argument "discriminator" which will replace
the value of mapper.polymorphic_on for that
query.  Mappers themselves no longer require
polymorphic_on to be set, even if the mapper
has a polymorphic_identity.   When not set,
the mapper will load non-polymorphically
by default. Together, these two features allow
a non-polymorphic concrete inheritance setup
to use polymorphic loading on a per-query basis,
since concrete setups are prone to many
issues when used polymorphically in all cases.