Add recommendation to use Content-Disposition header with AtomPub setContentStream operation

Using AtomPub, the setContentStream call uses PUT and does not specify a way to pass the filename.

A recommendation should be added to the spec to use the Content-Disposition header to transport the file name from the client to the repository.

See http://oasis-open.org/apps/org/workgroup/cmis/email/archives/201102/msg00018.html