[BUG] mel-q's encoder doesn't work with Emacs 23

[Katsumi Yamaoka <yamaoka@jpl.org>]

Hi,

I realized mel-q.el's QP encoder doesn't work with Emacs23 if it
runs in a multibyte buffer.  Some people seem to use it rather
than the one of mel-q-ccl.el for some reason.

Cf. http://article.gmane.org/gmane.mail.wanderlust.general/2695

Related articles:
http://article.gmane.org/gmane.emacs.mime.japanese/676
http://article.gmane.org/gmane.emacs.mime.japanese/677 (Japanese)

The way to reproduce the bug is as follows:

--8<---------------cut here---------------start------------->8---
(require 'mel)
(mel-use-module 'mel-q '("quoted-printable" "Q"))

(let ((buffer "*testing*"))
  (pop-to-buffer buffer)
  (erase-buffer)
  (set-buffer-multibyte t)
  (insert "Grüß Gott\n")
  (sit-for 1)
  (encode-coding-region (point-min) (point-max) 'iso-8859-1)
  (sit-for 1)
  (funcall (mel-find-function 'mime-encode-region "quoted-printable")
	   (point-min) (point-max)))
--8<---------------cut here---------------end--------------->8---

Debugger entered--Lisp error: (args-out-of-range "0123456789ABCDEF" 262143)
[...]

262143 is a representation of the iso-8859-1 encoded letter "ü"
in the multibyte buffer of Emacs 23, though it is displayed as
"\374".  In Emacs 22 or older, it is just 252 (i.e., ?\374).
The attached patch makes mel-q's encoder run in a unibyte buffer.

--- mel-q.el~	2005-07-06 02:09:04 +0000
+++ mel-q.el	2009-11-25 00:09:48 +0000
@@ -44,6 +44,16 @@
    (char-to-string (aref quoted-printable-hex-chars (logand character 15)))))
 
 (defun quoted-printable-internal-encode-region (start end)
+  (let ((data (buffer-substring start end)))
+    (delete-region (goto-char start) end)
+    (insert
+     (with-temp-buffer
+       (set-buffer-multibyte nil)
+       (insert data)
+       (quoted-printable-internal-encode-region-1 (point-min) (point-max))
+       (buffer-string)))))
+
+(defun quoted-printable-internal-encode-region-1 (start end)
   (save-excursion
     (save-restriction
       (narrow-to-region (goto-char start) end)