Pass values through get_db_prep_save() in a QuerySet.update() call.

This removes a long-standing FIXME in the update() handling and allows for
greater flexibility in the values passed in. In particular, it brings updates
into line with saves for django.contrib.gis fields, so fixed #10411.

Thanks to Justin Bronn and Russell Keith-Magee for help with this patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10003 bcc190cf-cafb-0310-a4f2-bffc1f526a37