Merge: Changed nitcorn HTTPResponse body type to Writable

This PR allows to send binaries (and whatever writable document) as bodies in nitcorn's responses.

The reduced use of buffers and copies should also reduce the pressure on the GC and make things faster.

Embrace, extend and extinguish #2703

Pull-Request: #2704
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>