I don't know about message/http, but I suspect it would need its own
rules. And it isn't clear this issue exists for HTTP anyhow, or that if
it does it should be solved this way.
I am not aware of a message/http. It is my understanding that if you have
an e-mail message attachment (with headers) that you should use the
message/rfc822 and then text/http as a MIME part of the attached message
(with multipart/mixed or multipart/alternative in the message header).
The concept of "message" is a lot more general than e-mail these days. As
are the uses MIME is put to.
Message/http is the media type for an HTTP message. See RFC2068 for details.
In general message/rfc822 is to be used to signify that a MIME part is a
message with headers and that the user should reference the header of that
message for it's context.
An email message, yes. A message in a more general sense, no.
Ned