bug#11514: 24.0.97; gnus-user-agent no longer sets up Gcc

[Romain Francoise]

If you put the following in /tmp/foo.el:

(setq mail-user-agent 'gnus-user-agent
      gnus-message-archive-group "nnimap+foo:Sent")

then start Emacs with `emacs -Q -l /tmp/foo.el' then do `C-x m', the
resulting Message buffer doesn't have a Gcc header. In Emacs 23 and
24.0.92, the Gcc header was (correctly, I think) added to the buffer.

Note: whether or not Gnus is started beforehand doesn't make a difference.