It seems that rexml/encodings/SHIFT-JIS.rb is broken. 
This is the patch for it.

diff -uNr rexml.org/encodings/SHIFT-JIS.rb rexml/encodings/SHIFT-JIS.rb
--- rexml.org/encodings/SHIFT-JIS.rb    2005-04-24 22:12:10.000000000
+0900
+++ rexml/encodings/SHIFT-JIS.rb        2005-04-24 22:10:36.000000000
+0900
@@ -3,11 +3,11 @@
 module REXML
   module Encoding
     def decode_sjis content
-      Uconv::u8tosjis(content)
+      Uconv::sjistou8(content)
     end

     def encode_sjis(str)
-      Uconv::sjistou8(str)
+      Uconv::u8tosjis(str)
     end

     b = proc do |obj|


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