test: failing multibyte unicode test

From #2410

test/xml/test_c14n.rb

@@ -1,3 +1,4 @@
+# coding: utf-8
 # frozen_string_literal: true
 
 require "helper"
@@ -237,6 +238,14 @@ def test_wrong_params
         assert_raises(TypeError) { doc.canonicalize(nil, :wrong_type) }
         doc.canonicalize(nil, nil, :wrong_type)
       end
+
+      def test_multibyte_unicode
+        # https://github.com/sparklemotion/nokogiri/issues/2410
+        doc = Nokogiri.XML(%{<foo>𡏅</foo>}, nil, "EUC-JP")
+        expected = %{<foo>陝</foo>}
+        result = doc.canonicalize
+        assert_equal(expected, result)
+      end
     end
   end
 end