Fix stack level too deep when model does not have attributes method.

Without that patch when using ActiveModel::AttributeMethods
in a class that does not respond to `attributes` method,
stack level too deep error will be raised on non existing
method. While documentation is clear that you need to define
`attributes` method in order to use AttributeMethods module,
`stack level too deep` is rather obscure and hard to debug,
therefore we should try to not break `method_missing` if
someone forgets about defining `attributes`.

activemodel/lib/active_model/attribute_methods.rb

@@ -394,7 +394,7 @@ def respond_to?(method, include_private_methods = false)
 
     protected
       def attribute_method?(attr_name)
-        attributes.include?(attr_name)
+        respond_to_without_attributes?(:attributes) && attributes.include?(attr_name)
       end
 
     private

activemodel/test/cases/attribute_methods_test.rb

@@ -76,7 +76,15 @@ def attribute(name)
   end
 end
 
+class ModelWithouAttributesMethod
+  include ActiveModel::AttributeMethods
+end
+
 class AttributeMethodsTest < ActiveModel::TestCase
+  test 'method missing works correctly even if attributes method is not defined' do
+    assert_raises(NoMethodError) { ModelWithouAttributesMethod.new.foo }
+  end
+
   test 'unrelated classes should not share attribute method matchers' do
     assert_not_equal ModelWithAttributes.send(:attribute_method_matchers),
                      ModelWithAttributes2.send(:attribute_method_matchers)