bug#30060: mh-send-letter coding-system selection

[Glenn Morris]

Package: emacs,mh-e
Version: 26.0.90
Severity: minor

mh-send-letter contains the following for selecting a coding system:

        (coding-system-for-write
         (if (and (local-variable-p 'buffer-file-coding-system
                                    (current-buffer)) ;XEmacs needs two args
                  ;; We're not sure why, but buffer-file-coding-system
                  ;; tends to get set to undecided-unix.
                  (not (memq buffer-file-coding-system
                             '(undecided undecided-unix undecided-dos))))
             buffer-file-coding-system
           (or (and (boundp 'sendmail-coding-system) sendmail-coding-system)
               (and (default-boundp 'buffer-file-coding-system)
                    (default-value 'buffer-file-coding-system))
               'iso-latin-1))))

This is similar to, but not quite the same as, the standard Emacs
function select-message-coding-system that exists specifically for this
purpose. Eg the mh-e version does not consult default-sendmail-coding-system.

It would be great if mh-e could use the standard function, or if not
have some comments added to the code explaining the reason(s) for the
difference(s).

Thanks.