Fixed #724 -- Ensured get_next_by_FOO() and get_previous_by_FOO() methods don't skip or duplicate any records in the case of duplicate values. Thanks for reporting the bug, mattycakes@gmail.com

git-svn-id: http://code.djangoproject.com/svn/django/trunk@1155 bcc190cf-cafb-0310-a4f2-bffc1f526a37