Rack rack

I have encountered a problem with the Rack::Session::Pool middleware. It creates a Rack::Response object which iterates over the body. Large files send by Static (underlying middleware) will be read to memory. There should be a possibility to disable the buffering. Buffering makes only sense if the middleware modifies the body.

Possibilities to fix this:
- Use Rack::Response object only if buffering is needed. Don't use it in core middlewares like Session::Pool. - Add an option to disable buffering and use the option in the core middlewares. - Split response class into two classes. One simple only for header manipulation and one extended with buffering.