Change HWIA#stringify_keys to return a HWIA not a Hash

activesupport/lib/active_support/hash_with_indifferent_access.rb

@@ -112,7 +112,7 @@ def delete(key)
     end
 
     def stringify_keys!; self end
-    def stringify_keys; to_hash end
+    def stringify_keys; dup end
     undef :symbolize_keys!
     def symbolize_keys; to_hash.symbolize_keys end
     def to_options!; self end

activesupport/test/core_ext/hash_ext_test.rb

@@ -61,6 +61,7 @@ def test_stringify_keys!
   end
 
   def test_symbolize_keys_for_hash_with_indifferent_access
+    assert_instance_of Hash, @symbols.with_indifferent_access.symbolize_keys
     assert_equal @symbols, @symbols.with_indifferent_access.symbolize_keys
     assert_equal @symbols, @strings.with_indifferent_access.symbolize_keys
     assert_equal @symbols, @mixed.with_indifferent_access.symbolize_keys
@@ -83,12 +84,14 @@ def test_symbolize_keys_preserves_fixnum_keys_for_hash_with_indifferent_access
   end
 
   def test_stringify_keys_for_hash_with_indifferent_access
+    assert_instance_of ActiveSupport::HashWithIndifferentAccess, @symbols.with_indifferent_access.stringify_keys
     assert_equal @strings, @symbols.with_indifferent_access.stringify_keys
     assert_equal @strings, @strings.with_indifferent_access.stringify_keys
     assert_equal @strings, @mixed.with_indifferent_access.stringify_keys
   end
 
   def test_stringify_keys_bang_for_hash_with_indifferent_access
+    assert_instance_of ActiveSupport::HashWithIndifferentAccess, @symbols.with_indifferent_access.dup.stringify_keys!
     assert_equal @strings, @symbols.with_indifferent_access.dup.stringify_keys!
     assert_equal @strings, @strings.with_indifferent_access.dup.stringify_keys!
     assert_equal @strings, @mixed.with_indifferent_access.dup.stringify_keys!