Use iso8601 time format in rspec expectation

spec/support/examples/a_create_endpoint.rb

@@ -87,7 +87,7 @@ def body_from_instance(instance, extra_params)
       end
       it "assigns the correct object to @#{factory.to_s}" do
         expect(assigns(factory)).to be_a model_class
-        expect(assigns(factory).created_at.to_formatted_s(:dpn)).to eql(valid_post_body[:created_at])
+        expect(assigns(factory).created_at.iso8601).to eql(valid_post_body[:created_at])
       end
       it "renders json" do
         expect(response.content_type).to eql("application/json")