bug#50752: 28.0.50; easy-menu-define lowers the menu-bar key

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Tue, 19 Oct 2021 05:52:15 +0200
> Cc: Shuguang Sun <shuguang79@qq.com>, 50752@debbugs.gnu.org
> 
> Which just looks wrong to me.  The logic in Fdowncase seems to assume
> that a unibyte string can never be downcased to a multibyte one, but
> that make_char_unibyte is picking out the lowest eight bits of the char
> we've downcased to and puts that into the downcased string.

What would you do instead?  It's a conundrum with no easy solutions.
The original string could include non-ASCII bytes.