bug#13330: 24.2; rmail-set-header doesn't work for multi-line headers

[Mark Lillibridge <mdl@alum.mit.edu>]

This should be obvious from inspection (rmail.el:2166):

  (defun rmail-set-header-1 (name value)
    "Subroutine of `rmail-set-header'.
  Narrow to header, set header NAME to VALUE, replacing existing if present.
  VALUE nil means to remove NAME altogether."
    (if (search-forward "\n\n" nil t)
        (progn
  	(forward-char -1)
  	(narrow-to-region (point-min) (point))
  	(goto-char (point-min))
  	(if (re-search-forward (concat "^" (regexp-quote name) ":") nil 'move)
              (if value
                  (progn
                    (delete-region (point) (line-end-position))
                    (insert " " value))
                (delete-region (line-beginning-position)
                               (line-beginning-position 2)))
            (if value (insert name ": " value "\n"))))
      (rmail-error-bad-format)))
  
  (defun rmail-set-header (name &optional msgnum value)
    "Set message header NAME to VALUE in message number MSGNUM.
  If MSGNUM is nil, use the current message.  NAME and VALUE are strings.
  VALUE may also be nil, meaning to remove the header."
    (rmail-apply-in-message msgnum 'rmail-set-header-1 name value)
    (with-current-buffer rmail-buffer
      ;; Ensure header changes get saved.
      ;; (Note replacing a header with an identical copy modifies.)
      (set-buffer-modified-p t)
      ;; However: don't save in mbox format over a Babyl file
      ;; merely because of this.
      (rmail-dont-modify-format)))

but if not, evaluate (esc-:) "(rmail-set-header "subject" nil)" on a
Rmail message with a multiline subject like:

|X-Spam-Status: No, score=-4.7 required=5.0 tests=BAYES_00,RCVD_IN_DNSWL_MED,
|	RP_MATCHES_RCVD autolearn=unavailable version=3.3.1
|From: Mark Lillibridge <mark.lillibridge@hp.com>
|To: <bug-gnu-emacs@gnu.org>
|Subject: this is a long subject
|  with several lines
|  indeed
|Date: Sat, 22 Oct 2011 12:05:57 -0700
|Reply-To: <mark.lillibridge@hp.com>
|Content-type: text/plain; charset=us-ascii
|
|
|empty body

The result (after hitting t twice to re-decode) is:

|X-Spam-Status: No, score=-4.7 required=5.0 tests=BAYES_00,RCVD_IN_DNSWL_MED,
|	RP_MATCHES_RCVD autolearn=unavailable version=3.3.1
|From: Mark Lillibridge <mark.lillibridge@hp.com>
|To: <bug-gnu-emacs@gnu.org>
|  with several lines
|  indeed
|Date: Sat, 22 Oct 2011 12:05:57 -0700
|Reply-To: <mark.lillibridge@hp.com>
|Content-type: text/plain; charset=us-ascii
|
|
|empty body

Notice that only the first line of the Subject has been deleted.  

    Should there be a warning that this function does not RFC2047 encode
value so the caller is responsible for doing any such encoding if
needed?

- Mark