[1.1.X] Fixed #13259 -- Ensure that multiple calls to message() don't corrupt...

[1.1.X] Fixed #13259 -- Ensure that multiple calls to message() don't corrupt any extra message headers. Thanks to canburak for the report, and Andi Albrecht for the fix.

Backport of r12901 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12902 bcc190cf-cafb-0310-a4f2-bffc1f526a37