When a Packer instance has an ext handler and it is passed None, it passes it to said handler instead of turning it into "\xc0" like it should. Same when None is inside an array. This doesn't happen when unpacking, when there is no handler, or with any other value AFAICT.

I reproduced it on Travis on all Pythons: https://travis-ci.org/lourkeur/libmpack-python/builds/229764123