Fixed #9364 -- now uses the appropriate database table for inherited...

Fixed #9364 -- now uses the appropriate database table for inherited `GeometryField`s; now uses the `SpatialBackend` booleans in tests.

Backport of r9336 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9337 bcc190cf-cafb-0310-a4f2-bffc1f526a37