Intercept contains_eager() with of_type, set aliased / polymorphic

Intercept contains_eager() with of_type, set aliased / polymorphic

Fixed bug in :func:`.contains_eager` query option where making use of a
path that used :meth:`.PropComparator.of_type` to refer to a subclass
across more than one level of joins would also require that the "alias"
argument were provided with the same subtype in order to avoid adding
unwanted FROM clauses to the query; additionally,  using
:func:`.contains_eager` across subclasses that use :func:`.aliased` objects
of subclasses as the :meth:`.PropComparator.of_type` argument will also
render correctly.

Change-Id: Ie1c10924faa45251aab1a076a3ba7ef9fb1bdeee
Fixes: #4130