Fixed #8023 -- Allow filtering of DecimalFields (in models) using strings.

At the same time, checked all other cases of db_prep_value() to ensure they
aren't making the same mistake.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8143 bcc190cf-cafb-0310-a4f2-bffc1f526a37