Hi,

I noticed this when I forwarded a mail using Gnus.  The problem
is that non-ASCII letters contained in the header of the original
mail will not be encoded properly.  The rough outline of what
`mml-generate-mime-1' does then are:

・Insert the original mail (decoded) into a unibyte buffer
  (that with-temp-buffer of mml.el-[line:619] generates).
・Detect the charset of the contents.
・Encode the contents by that detected charset.
・Insert the encoded contsnts into the other unibyte buffer
  (that with-temp-buffer of mml.el-[line:612] generates).
・Run `mml-to-mime' that encodes the header of the original mail
  using `mail-encode-encoded-word-buffer' (an alias to
  `rfc2047-encode-message-header').

rfc2047's encoder expects human readable text, however, there
are encoded ones as described above.  So, as for at least mail
forwarding, `mml-generate-mime-1' is seriously broken.

The current code looks like WIP to me.  How about reverting it
to the emacs-25 version?  Slightly modernized[1] one is attached.

[1] Don't use old compat functions and string-as-(multi|uni)byte.

In GNU Emacs 26.0.50 (build 1, i686-pc-cygwin, GTK+ Version 3.18.9)
 of 2017-05-30 built on localhost
Windowing system distributor 'The Cygwin/X Project', version 11.0.11900000