not required, by default), set the default properly in the constructor.

This code can be simplified when/if we resolve the
BooleanField/NullBooleanField overlap, but the current stuff is backwards
compatible. This would previously cause SQL errors on PostgreSQL and
interesting failures in subtle ways on MySQL and SQLite.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8050 bcc190cf-cafb-0310-a4f2-bffc1f526a37