(rmail-redecode-body): Don't encode/decode the message; instead, just rewrite

(rmail-redecode-body): Don't encode/decode the message; instead, just rewrite
the X-Coding-System header with the new encoding, and let rmail-show-message
do the rest.  Remove unused argument RAW.  Fix doc string to be consistent
with the new implementation.
(rmail-show-message): Honor X-Coding-System header, if present, in preference
to Content-Type header.