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

[Eli Zaretskii]

> From: Markus Rost <rost@math.uni-bielefeld.de>
> CC: 4623@emacsbugs.donarmstrong.com
> Date: Sun,  6 Dec 2009 21:23:48 +0100 (CET)
> 
> One minor thing:  Since rmail-message-encoding is a global variable,
> several Rmail buffers may interfere.

Right, I didn't think abut that possibility.  I will try to think
about some simple solution.

Thanks for testing.  Unless someone objects, I will commit these
changes (suitably cleaned up) by today's evening.