change PWP::Encoding to PWP::SingleEncoding

Hi,

Starting from 4.000 `Pod::Weaver` includes `SingleEncoding` plugin (it defaults to 'UTF-8'), which makes `Pod::Weaver::Plugin::Encoding` an extra dependency that could be thrown away.

See [Rik's post](http://rjbs.manxome.org/rubric/entry/2021) and also check out my quest for details/comments:

> http://questhub.io/realm/perl/quest/5277f3cc9f567ad56f0000e7

Cheers,
Sergey