[1.2.X] Fixed #14198 -- Corrected rendering of generic sitemaps when no...

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

Backport of r13676 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13677 bcc190cf-cafb-0310-a4f2-bffc1f526a37