non_primary mapper should maintain identity_class of the primary

[Mike Bayer]

class AddressUser(User):
    pass
m1 = mapper(User, users)
m2 = mapper(AddressUser, addresses, inherits=User)
m3 = mapper(AddressUser, addresses, non_primary=True)
assert m3._identity_class is m2._identity_class

else non-compatible AddressUser classes are created

[Mike Bayer]

7eb29add0557371020a55fecc82d7d4c45575fcb 7df21e5a1f252ff768f684132db95bd1b5e78efa

[Mike Bayer]

Removing milestone: 0.6.8 (automated comment)