[1.0.X] Fixed #5903 -- DecimalField.get_default() now correctly returns a...

[1.0.X] Fixed #5903 -- DecimalField.get_default() now correctly returns a Decimal object when the model instance was not retrieved from the database. Thanks Justin Driscoll and pigletto.

Backport of r9823 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9824 bcc190cf-cafb-0310-a4f2-bffc1f526a37