Only convert direct hash instances in hash with indifferent access.

activesupport/lib/active_support/hash_with_indifferent_access.rb

@@ -140,11 +140,10 @@ def convert_key(key)
       end
 
       def convert_value(value)
-        case value
-        when Hash
+        if value.class == Hash
           self.class.new_from_hash_copying_default(value)
-        when Array
-          value.dup.replace(value.collect { |e| e.is_a?(Hash) ? self.class.new_from_hash_copying_default(e) : e })
+        elsif value.is_a?(Array)
+          value.dup.replace(value.map { |e| convert_value(e) })
         else
           value
         end

activesupport/test/core_ext/hash_ext_test.rb

@@ -12,6 +12,9 @@ class IndifferentHash < HashWithIndifferentAccess
   class SubclassingArray < Array
   end
 
+  class SubclassingHash < Hash
+  end
+
   def setup
     @strings = { 'a' => 1, 'b' => 2 }
     @symbols = { :a  => 1, :b  => 2 }
@@ -105,6 +108,11 @@ def test_stringify_keys_bang_for_hash_with_indifferent_access
     assert_equal @strings, @mixed.with_indifferent_access.dup.stringify_keys!
   end
 
+  def test_hash_subclass
+    flash = { "foo" => SubclassingHash.new.tap { |h| h["bar"] = "baz" } }.with_indifferent_access
+    assert_kind_of SubclassingHash, flash["foo"]
+  end
+
   def test_indifferent_assorted
     @strings = @strings.with_indifferent_access
     @symbols = @symbols.with_indifferent_access