Add test to demonstrate issue deserializing hash with ivar

Currently the elements of a hash are revived before any ivar values.
This causes an issue when the `[]=` method references an instance
variable.

test/psych/test_hash.rb

@@ -6,6 +6,18 @@ module Psych
     class X < Hash
     end
 
+    class HashWithIvar < Hash
+      def initialize
+        @keys = []
+        super
+      end
+
+      def []=(k, v)
+        @keys << k
+        super(k, v)
+      end
+    end
+
     class HashWithCustomInit < Hash
       attr_reader :obj
       def initialize(obj)
@@ -24,6 +36,14 @@ module Psych
       @hash = { :a => 'b' }
     end
 
+    def test_hash_with_ivar
+      t1 = HashWithIvar.new
+      t1[:foo] = :bar
+      t2 = Psych.load(Psych.dump(t1))
+      assert_equal t1, t2
+      assert_cycle t1
+    end
+
     def test_referenced_hash_with_ivar
       a = [1,2,3,4,5]
       t1 = [HashWithCustomInit.new(a)]