bug#15195: 24.3.50; message-make-in-reply-to out of compliance with RFC2822

[Aaron Ecay]

The `message-make-in-reply-to' function generates In-Reply-To headers
of the form:
"<msg@id> (John Smith's message of DATE)"

However, RFC2822 (pp. 22-23) states that the In-Reply-To field should
contain only 1 msg-id token (possibly more than one in some cases), but
not any other material.

The following implementation of the function inserts just the parent
message’s ID, in compliance with the RFC:
,----
| (defun message-make-in-reply-to ()
|   "Return the In-Reply-To header for this message."
|   (when message-reply-headers
|     (mail-header-id message-reply-headers)))
`----

-- 
Aaron Ecay