bug#28266: 25.2; RMAIL FCC coding issue

[charles@aurox.ch (Charles A. Roelli)]

From emacs -q (25.2 or 26.0.50):

M-: (setq mail-user-agent 'sendmail-user-agent) RET
C-x m
someone@example.com C-n
test RET
FCC: /tmp/emacs-bug-mbox
C-u 2 C-n
This is a test mail, containing ‘’ quotes.
C-c C-c
transport RET ; On my system, this doesn't actually send the mail, so
	      ; this is the mail-sending system I choose to reproduce
	      ; the issue.  In any case, the recipient email doesn't
	      ; really exist so it should be okay to choose this.
C-u M-x rmail /tmp/emacs-bug-mbox ; Observe correct message contents.
C-x o ; Switch back to the *mail* buffer.
C-c C-c y ; Resend it.
n ; Open the resent copy.

This is RMAIL's copy of the resent mail:

----------
Date: Mon, 28 Aug 2017 20:41:35 +0200
From: charles@localhost (Charles A. Roelli)
To: someone@example.com
Subject: test
Content-type: text/plain; charset=utf-8

This is a test mail, containing ^X^Y quotes.
----------

Notice that the quotes have been converted to ^X/^Y
characters (characters have been corrupted).  I tested with
another non-ASCII character (é), and it got converted to \351.

The same issue does not occur with the default value of
mail-user-agent (`message-user-agent').