IHttpResponse in Document.save()

We're using the preview version of Microsoft's MVC package.

This may be a dumb question, but I have the IHttpResponse, but the doc.Save() method only takes HttpResponse object.

I can't cast the IHttpResponse to HttpResponse, and I really don't want to have to save the document to the file system. I need the user to be able to download the generated file on the fly.

Any thoughts?