Use dup to preserve previous behavior

activesupport/lib/active_support/hash_with_indifferent_access.rb

@@ -11,7 +11,7 @@ module ActiveSupport
     end
 
     def with_indifferent_access
-      self
+      dup
     end
 
     def initialize(constructor = {})

activesupport/test/core_ext/hash_ext_test.rb

@@ -971,7 +971,7 @@ class HashToXmlTest < Test::Unit::TestCase
     assert_nil hash_wia.default
   end
 
-  def test_should_return_self_for_with_indifferent_access
+  def test_should_return_dup_for_with_indifferent_access
     hash_wia = HashWithIndifferentAccess.new
     assert_equal hash_wia, hash_wia.with_indifferent_access
   end