mm-encode doesn't know message/rfc822, and base64's them

[Karl Kleinpaste <karl@justresearch.com>]

I wanted to send a message containing a message/rfc822 pulled from my
own archives, but `C-c C-a' didn't understand "message/rfc822" as a
valid type.  How do we get more types into the available set?  A quick
look through mailcap.el didn't seem to tell me what I wanted to know,
because there are a lot of types there which don't show up in
*Completions* when I try to TAB-complete at the `C-c C-a' prompt.

I can stuff in a message as text/plain initially, and then manually
change it to message/rfc822 in the "part" spec.  But then mm-encode.el
wants to base64-encode it, because the CTE defaults aren't terribly
specific.  I did this to mm-encode.el:

1998-12-07  Karl Kleinpaste  <karl@justresearch.com>

	* mm-encode.el (mm-content-transfer-encoding-defaults): Add an
	entry for message/rfc822 as 8bit.

--- mm-encode.el.~1~	Mon Dec  7 16:10:26 1998
+++ mm-encode.el	Mon Dec  7 16:10:46 1998
@@ -29,6 +29,7 @@
 
 (defvar mm-content-transfer-encoding-defaults
   '(("text/.*" quoted-printable)
+    ("message/rfc822" 8bit)
     ("application/emacs-lisp" 8bit)
     (".*" base64))
   "Alist of regexps that match MIME types and their encodings.")

But I suspect that something much more general is really in order
here.  And I don't know whether 8bit is appropriate, or if perhaps
quoted-printable is what really ought to be used.