change PWP::Encoding to PWP::SingleEncoding

Be sure to let me know if you're tired of getting [those](https://github.com/kentfredric/Dist-Zilla-Plugin-Prereqs-MatchInstalled-All/pull/1) [dull](https://github.com/kentfredric/Dist-Zilla-Util-BundleInfo/pull/1) [requests](https://github.com/kentfredric/Dist-Zilla-Plugin-Prereqs-Plugins/pull/1) :)