Merge: nitcorn: fix bug with binary files

Downloading binary files from a nitcorn server works as expected once again.

The API should be enough for most users but there is a few features lacking. Namely alternating custom text with files and writing custom data. We could merge the `body` and `files` attributes into a sequence of `Bytables` to send, with support for `Bytes`.

You can test the result on xymus.net, either by downloading the latest WBTW at http://xymus.net/pub/wbtw-v0.4-110-g668b261.tar.gz or by playing a clone of Super Hexagon (mostly made by @ablondin) at http://xymus.net/pub/hex/.

Pull-Request: #1817
Reviewed-by: Jean Privat <jean@pryen.org>