[plt-scheme] date header for mail messages

When I use the `smtp' and `head' libraries from the `net' collection,
the mail that I send gets these headers added along the way:

X-Comment: Sending client does not conform to RFC822 minimum requirements
X-Comment: Date has been added by Maillennium

I'm using `standard-message-header', but I guess that doesn't include
the date, which is required by RFC822.  Should it?  Or should
`smtp-send-message' add the date?  Or should the user be responsible
for adding it to the result of `standard-message-header'?

I looked at the `date' library in MzLib, but I couldn't find a date
format that matched RFC822's.  Is there some standard library that
produces this format?  I found some code in the `web-server'
collection that seems to do the right thing, but it's not exported.

--dougo at ccs.neu.edu