Can't set headers per part of multipart requests

I think it would be helpful to allow users to specify per-part headers somewhere in this code.

As it is now, I cannot specify a Content-Location header per part, which makes it impossible to encapsulate subsidiary parts of aggregate documents (whose Content-Location can be different per part) -- see http://tools.ietf.org/html/rfc2557.

Can anyone thing of any major objects to adding this functionality here