bug#4623: 23.1.50; rmail changes encoding of characters on save

[Eli Zaretskii]

> From: Stefan Monnier <monnier@IRO.UMontreal.CA>
> Cc: 4623@emacsbugs.donarmstrong.com
> Date: Mon, 07 Dec 2009 14:25:37 -0500
> 
> > Well, I did precisely that, as you could see, but then Markus pointed
> > out that a single global variable will not work when there are more
> > than one Rmail buffer.
> 
> Of course, just like buffer-file-coding-system, it should be buffer-local.

Yes, I thought about that at first.  But this solution complicates
things because of the swapping.  So I went for a slightly different
one, which stashes the encoding immediately prior to switching to the
``other'' buffer.  This makes sure we have always the correct
encoding.