Added a missing warning forgotten in r17392.

        if isinstance(value, datetime.date):

            value = datetime.datetime(value.year, value.month, value.day)

            if settings.USE_TZ:

                # For backwards compatibility, interpret naive datetimes in

                # local time. This won't work during DST change, but we can't

                # do much about it, so we let the exceptions percolate up the

                # call stack.

                warnings.warn(u"DateTimeField received a naive datetime (%s)"

                              u" while time zone support is active." % value,

                              RuntimeWarning)

                default_timezone = timezone.get_default_timezone()

                value = timezone.make_aware(value, default_timezone)

            return value