Fixed #14198 -- Corrected rendering of generic sitemaps when no priority is...

Fixed #14198 -- Corrected rendering of generic sitemaps when no priority is specified. Thanks to palkeo for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13676 bcc190cf-cafb-0310-a4f2-bffc1f526a37