bug#25645: 26.0.50; message.el: Fcc+handler results in write coding issues in emacs 26

[Eli Zaretskii <eliz@gnu.org>]

> From: Yuri D'Elia <wavexx@thregr.org>
> Date: Tue, 07 Feb 2017 18:00:01 +0100
> 
> Somehow when the mail is encoded in quoted printable with an utf-8
> charset, the buffer seem to contain raw bytes as if the buffer has
> an incorrect buffer-file-coding-system.

Buffer contents is entirely unrelated to the value of
buffer-file-coding-system.  The latter only determines how to encode
the buffer contents when writing it to a file or sending it to another
process or network connection.

If you get raw bytes in the buffer, the culprit is in the code which
inserted those bytes, or read them from some file.

> When 'write-file is issued, emacs prompts for a coding system, which is
> unexpected.

This is expected when there are raw bytes in the buffer.  The problem
is elsewhere.