* lib/rexml/encoding.rb: use Ruby native encoding mechnism. [ruby-dev:42464]
* lib/rexml/encodings/: remove.
* lib/rexml/document.rb, lib/rexml/formatters/default.rb,
lib/rexml/output.rb, lib/rexml/parseexception.rb,
lib/rexml/parsers/baseparser.rb, lib/rexml/source.rb,
lib/rexml/xmldecl.rb: use Ruby's native Encoding object.
* test/rexml/, test/rss/: follow the above encoding chagnes.
* NEWS: add REXML's incompatible change about encoding.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@29646 b2dd03c8-39d4-4d8f-98ff-823fe69b080e