new Chinese GBK transcoding (from Yoshihiro Kambayashi) * test/ruby/test_transcode.rb: added tests for the above (from Yoshihiro Kambayashi) git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@21315 b2dd03c8-39d4-4d8f-98ff-823fe69b080e