run `gnus-before-do-gcc-hook'.
        (message-narrow-to-headers)
        (let ((gcc (or gcc (mail-fetch-field "gcc" nil t)))
              (cur (current-buffer))
+             (coding-system-for-write 'raw-text)
              groups group method)
          (when gcc
            (message-remove-header "gcc")
              (save-excursion
                (nnheader-set-temp-buffer " *acc*")
                (insert-buffer-substring cur)
+               (run-hooks 'gnus-before-do-gcc-hook)
                (goto-char (point-min))
                (when (re-search-forward
                       (concat "^" (regexp-quote mail-header-separator) "$")