[1.2.X] Fixed #13987 -- Ensure that the primary key is set correctly for all...

[1.2.X] Fixed #13987 -- Ensure that the primary key is set correctly for all models that have concrete-abstract-concrete inheritance, not just the first model. Thanks to Aramgutang for the report and patch.

Backport of r15498 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15499 bcc190cf-cafb-0310-a4f2-bffc1f526a37