change PWP::Encoding to PWP::SingleEncoding

Hi,

Pod::Weaver now has [-SingleEncoding] plugin (starting from 4.000), which makes Pod::Weaver::Plugin::Encoding an extra dependency that could be thrown away.

See my quest for details/comments:

> http://questhub.io/realm/perl/quest/5277f3cc9f567ad56f0000e7

Cheers,
Sergey