Fixed #9926 -- Fixes for some select_related() situations.

Using select_related(...) across a nullable relation to a multi-table
model inheritance situation no longer excludes results. Thanks to AdamG
for a test demonstrating part of the problem.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10136 bcc190cf-cafb-0310-a4f2-bffc1f526a37