Add support for processing finished async-uploads

[paneq]

Branch: rack-adapter

Line from : M2R::Adapter::Rack

rack_input = StringIO.new(request.body)

This could be something like:

rack_input = request.body_io

Request would return StringIO.new(body) for this method.

But in case of async upload we would extend the request object with a module AsyncUploadRequest returning File or Tempfile instead.