Filenames shouldn't be URL-escaped in the Packages manifest

Per https://wiki.debian.org/RepositoryFormat#Filename. It looks like the
transport will then double-escape the URL, causing a 404.

Probably related to krobertson/deb-s3#44.
This method parses not only the package manifest, where it should indeed be nil, but also the deb's control file where it would be – hence the defensiveness :)