Fixed #12455 -- corrected an oversight in result_headers not honoring admin_order_field

This finishes some slightly refactoring that went into the admin_list
template tags.

Thanks to kegan for discovering the oversight.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12157 bcc190cf-cafb-0310-a4f2-bffc1f526a37