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

[Markus Rost]

> Here's what I came up with.

You are a Master of the Universe.  Once rmail-swap-buffers is set up
correctly, things get much simpler.

I tested:  The old bug is still gone, and problems which I encountered
during my own hacking don't show up.

One minor thing:  Since rmail-message-encoding is a global variable,
several Rmail buffers may interfere.  If you have displayed a message
in buffer A with encoding A (so rmail-message-encoding is set to A)
and you switch to a modified Rmail buffer B and save that buffer, then
the message in buffer B gets encoding A.  Only temporarily of course,
as before.  I don't suggest to fix that now.  Perhaps one could put a
note in the code.