bug#3790: 23.0.95; C-x C-s in RMAIL buffer clobbers buffer-file-coding-system

After typing C-x C-s in an Rmail buffer showing some message,
buffer-file-coding-system becomes no-conversion, no matter what
encoding was used to decode and display the message.  Typing `p' then
`n' to get the message redisplayed restores the correct value of
buffer-file-coding-system.

I suspect that no-conversion comes from the buffer where Rmail holds
the whole collection of messages.


In GNU Emacs 23.0.95.1 (i386-mingw-nt5.1.2600)
 of 2009-06-27 on HOME-C4E4A596F7
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (3.4)'

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: ENU
  value of $XMODIFIERS: nil
  locale-coding-system: cp1255
  default-enable-multibyte-characters: t

Major mode: RMAIL

Minor modes in effect:
  diff-auto-refine-mode: t
  shell-dirtrack-mode: t
  desktop-save-mode: t
  show-paren-mode: t
  display-time-mode: t
  tooltip-mode: t
  tool-bar-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  global-auto-composition-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  temp-buffer-resize-mode: t
  line-number-mode: t

Recent input:
- d o n e @ <switch-frame> <switch-frame> e m a c s 
b u g s . d o n a r m s t r o n g . c o m <down> <switch-frame> 
<down> <down> <down> <down> <down> <down> <C-right> 
<C-right> <C-right> <C-right> <C-right> <C-right> <C-left> 
C-SPC C-e M-w <switch-frame> C-y <C-end> D o n e <backspace> 
<backspace> <backspace> <backspace> F i x e d SPC ( 
s e e SPC <switch-frame> M-n M-n <switch-frame> # 3 
4 3 2 SPC f o r SPC d e t a i l s ) . <return> C-c 
C-s <switch-frame> p p d d d d <switch-frame> <switch-frame> 
<switch-frame> <help-echo> <switch-frame> M-1 g <up> 
<up> <return> SPC SPC SPC d d d C-x C-s <help-echo> 
<help-echo> M-p M-n M-p M-p M-p M-n M-n M-n <help-echo> 
C-x C-s <help-echo> <help-echo> <help-echo> M-1 g <up> 
<return> d SPC d C-z C-z C-z C-z C-z C-z C-z C-z C-z 
C-z C-z C-z C-z C-z C-z C-z C-z C-z C-z C-z C-z C-z 
C-z d SPC d SPC d <C-end> C-x C-s M-p M-n <help-echo> 
M-x f i n f <backspace> d - f i l e - l i <tab> <return> 
<up> <return> C-SPC <next> <next> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <up> C-x C-x C-x C-x C-x <return> c u n i <backspace> 
<backspace> <backspace> C-g C-x <return> c <return> 
M-x w r i t e - r e g <tab> <return> m a i l . o n 
e <backspace> <tab> <tab> e <return> C-x k <return> 
C-u g <up> <return> C-x C-s M-x r e p o r t - e m <tab> 
<help-echo> <help-echo> <help-echo> <return>

Recent messages:
Quit
Wrote d:/usr/eli/data/mail.one
Getting mail from d:/usr/eli/data/mail.one...
Counting new messages...done (1)
Saving file d:/usr/eli/rmail/INBOX...
Wrote d:/usr/eli/rmail/INBOX
Computing summary lines...done
1 new message read
Saving file d:/usr/eli/rmail/INBOX...
Wrote d:/usr/eli/rmail/INBOX