Fixed #10443 -- Fixed model attribute updating after r10003.

Adding a get_db_prep_save() call to the UpdateQuery code path meant it
was being called twice if you updated an existing model attribute. This
change removes that double call and also makes TimeField.to_python() a
little more robust for the benefit of the Oracle backend (just in case).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10013 bcc190cf-cafb-0310-a4f2-bffc1f526a37