[1.0.X] Fixed #9775 -- Fixed an oversight from r9601 and allow direct attribute

lookup in the serializable_value() method. This means that abstract
parents that are multi-table children of other models(no, really!!) now
work again.

Backport of r9618 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9619 bcc190cf-cafb-0310-a4f2-bffc1f526a37