Fix problems that can occur with legacy sites that already have multiple case-insensitive variations of a username.
Fix problems that can occur with legacy sites that already have multiple case-insensitive variations of a username.

aptivate/django-registration,AndrewLvov/django-registration,QPmedia/django-registration,AndrewLvov/django-registration,fedenko/django-registration,aptivate/django-registration,christang/django-registration-1.5,christang/django-registration-1.5,QPmedia/django-registration,siddharthsarda/django-registration,thedod/django-registration-hg-mirror,fedenko/django-registration