psych doesn't handle instance variables in subclasses of Hash

test_hash.patch

diff --git a/test/psych/test_hash.rb b/test/psych/test_hash.rb
index 4bd4edf..b000226 100644
--- a/test/psych/test_hash.rb
+++ b/test/psych/test_hash.rb
@@ -5,6 +5,10 @@ module Psych
     class X < Hash
     end
 
+    class Y < Hash
+      attr_accessor :val
+    end
+
     def setup
       super
       @hash = { :a => 'b' }
@@ -16,6 +20,12 @@ module Psych
       assert_equal X, x.class
     end
 
+    def test_subclass_with_attributes
+      y = Psych.load Psych.dump Y.new.tap {|y| y.val = 1}
+      assert_equal Y, y.class
+      assert_equal 1, y.val
+    end
+
     def test_map
       x = Psych.load "--- !map:#{X} { }\n"
       assert_equal X, x.class