Make generated_attribute_methods to private

Because `generated_attribute_methods` is an internal API.

activemodel/lib/active_model/attribute_methods.rb

@@ -326,11 +326,11 @@ def undefine_attribute_methods
         attribute_method_matchers_cache.clear
       end
 
-      def generated_attribute_methods #:nodoc:
-        @generated_attribute_methods ||= Module.new.tap { |mod| include mod }
-      end
-
       private
+        def generated_attribute_methods
+          @generated_attribute_methods ||= Module.new.tap { |mod| include mod }
+        end
+
         def instance_method_already_implemented?(method_name)
           generated_attribute_methods.method_defined?(method_name)
         end

activemodel/test/cases/attribute_methods_test.rb

@@ -116,7 +116,7 @@ class AttributeMethodsTest < ActiveModel::TestCase
 
   test "#define_attribute_method does not generate attribute method if already defined in attribute module" do
     klass = Class.new(ModelWithAttributes)
-    klass.generated_attribute_methods.module_eval do
+    klass.send(:generated_attribute_methods).module_eval do
       def foo
         "<3"
       end

activerecord/test/cases/attribute_methods_test.rb

@@ -1005,7 +1005,7 @@ def new_topic_like_ar_class(&block)
         class_eval(&block)
       end
 
-      assert_empty klass.generated_attribute_methods.instance_methods(false)
+      assert_empty klass.send(:generated_attribute_methods).instance_methods(false)
       klass
     end