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

[Yuri D'Elia <wavexx@thregr.org>]

On Tue, Feb 07 2017, Eli Zaretskii wrote:
> If you get raw bytes in the buffer, the culprit is in the code which
> inserted those bytes, or read them from some file.

Yes, the content of the buffer is generated by
message-encode-message-body in message.el itself.

You only need to insert an unicode letter in the message's buffer to
trigger the issue when recoding to qp-utf8.

I wrote to Lars prior to filing this report (cc-ed).

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

Since the fcc-handler is expected to do something with the buffer (write
it out), I expect message.el to preset the buffer coding system if the
buffer becomes unibyte in nature.