Fixed #7880 -- Corrected the handling of fields in the admin that specify...

Fixed #7880 -- Corrected the handling of fields in the admin that specify choices, so that the presence of choices overrides the decision to use a custom admin widget. This is primarily of interest to Date/Time fields. Thanks to camilonova for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8153 bcc190cf-cafb-0310-a4f2-bffc1f526a37