Fixed Queryset.dates() in the presence of extra-select columns.

Any extra(select=...) columns can be ignored in the SQL for dates, since we are
only interested in extracting distinct date values. We were previously
including them by accident and it was generating incorrect SQL.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@9091 bcc190cf-cafb-0310-a4f2-bffc1f526a37