bug#4624: 23.1.50; rmail-edit may add incorrect content-type header

In rmail look at a message (from an mbox file) with content-type
multipart, e. g., with headers like

Content-Type: multipart/alternative; boundary="bbbbbbbbbbb"

Edit that message within rmail as usual with
rmail-edit-current-message and rmail-cease-edit.  After saving, the
message has an *additional* header like

Content-type: text/plain; charset=us-ascii

This is bad, since mail readers may not handle the message correctly
as multipart anymore.

The reason is that rmail-cease-edit uses rmail-mime-charset-pattern to
identify content-type headers, but rmail-mime-charset-pattern will match
by default only text/plain.

I am not sure how to fix this.  Ideally rmail-cease-edit should walk
through all the parts of a message when handling the coding, but that
looks very complicated.  Maybe for now rmail-cease-edit should handle
the coding only if there is no content-type header or if the
content-type is matched by rmail-mime-charset-pattern, leaving the
other cases to the responsibilty of the user.


In GNU Emacs 23.1.50.1 (i686-pc-linux-gnu, GTK+ Version 2.16.1)
 of 2009-10-02 on laptop
Windowing system distributor `The X.Org Foundation', version 11.0.10600000
configured using `configure  '--prefix=/home/rost/local/cvs''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: C
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t