Fix class_attribute so that it works with singleton classes.

activesupport/lib/active_support/core_ext/class/attribute.rb

@@ -50,7 +50,7 @@ def class_attribute(*attrs)
         singleton_class.send(:define_method, attr) { value }
       end
 
-      define_method(attr) { self.class.send(attr) }
+      define_method(attr) { self.singleton_class.send(attr) }
       define_method(:"#{attr}?") { !!send(attr) }
       define_method(:"#{attr}=") do |value|
         singleton_class.remove_possible_method(attr)

activesupport/test/core_ext/class/attribute_test.rb

@@ -59,4 +59,10 @@ def setup
     object = Class.new { class_attribute :setting, :instance_writer => false }.new
     assert_raise(NoMethodError) { object.setting = 'boom' }
   end
+
+  test 'works well with singleton classes' do
+    object = @klass.new
+    object.singleton_class.setting = 'foo'
+    assert_equal 'foo', object.setting
+  end
 end