a String which means the name of the encoding.
  this partially revert r29646.

* lib/rexml/document.rb: follow above.

* lib/rexml/output.rb: ditto.

* lib/rexml/parsers/baseparser.rb: ditto.

* lib/rexml/source.rb: ditto.

* lib/rexml/xmldecl.rb: ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@31008 b2dd03c8-39d4-4d8f-98ff-823fe69b080e