[1.0.X] Fixed #10240 -- Restored YAML serialization with Decimal objects that...

[1.0.X] Fixed #10240 -- Restored YAML serialization with Decimal objects that broke with r9823. Thanks Alex Gaynor for the report.

Backport of r9825 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9826 bcc190cf-cafb-0310-a4f2-bffc1f526a37