bug#4816: change of coding system without inquiry

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Sun, 28 Feb 2016 17:37:13 +1100
> Cc: 4816@debbugs.gnu.org
> 
> I think this has probably been fixed in the intervening years

Actually, I very much doubt that.  We still silently use the preferred
encoding if the original one doesn't fit.

This is a rare use case.  It's possible that just saying

  (prefer-coding-system 'latin-1)

is all that's needed to get the required prompt back.

I think we should also welcome a patch to provide a user option that
will always force the prompt.