overriding different columns with the same key during inheritance

r4952 commits a basic proof of the issue, which looks like:

base = Table('base', metadata, 
    Column('base_id', Integer, primary_key=True),
    Column('data', String(255))
    )

subtable = Table('subtable', metadata,
    Column('base_id', Integer, ForeignKey('base.base_id'), primary_key=True),
    Column('subdata', String(255))
)

class Base(object):
    pass
class Sub(Base):
    pass

mapper(Base, base, properties={
    'id':base.c.base_id
})
mapper(Sub, subtable, inherits=Base, properties={
    'id':subtable.c.base_id
})

# what in fact happens is that "Sub" gets the column "base_id" mapped
# as well.   what *should* happen is, Sub mapper should reconcile
# the inherited "id" from Base.  right now Base's "id" is ignored
# totally because the same key is present in Sub.

self.assertEquals(
    class_mapper(Sub).get_property('id').columns,
    [subtable.c.base_id](base.c.base_id,)
)

# this fails too
s1 = Sub()
s1.id = 10
sess = create_session()
sess.add(s1)
sess.flush()
assert sess.query(Sub).get(10) is s1

this can come up pretty easily when using Declarative.

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