* lib/rexml/xmldecl.rb (REXML::XMLDecl): Stop using REXML::Encoding
module because XMLDecl doesn't convert encoding. This causes
removing XML encoding name normalization (encoding.upcase).
Encoding name in XML declaration is what user specifies.
I think this is reasonable change.
* test/rexml/test_xml_declaration.rb: Add tests for the above change.
* test/rexml/test_core.rb (Tester#test_ticket_88): Follow the
change that encoding name in XML declaration isn't normalized.