[1.0.X] Fixed #9233 -- Allow date and message-id headers to be passed in

manually in email messages. Previously we were creating duplicate headers,
which was bad.

Backport of r9197 from trunk. I'm calling this a bugfix, since the docs are
unclear about which headers you can pass into an EmailMessage constructor.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9198 bcc190cf-cafb-0310-a4f2-bffc1f526a37