Fix integration test.

Author: dblock

spec/issues/962_polymorphic_entity_with_custom_documentation_spec.rb

@@ -95,8 +95,7 @@ class EntityWithNestedEmptyEntity < Grape::Entity
   specify do
     expect(hidden_entity_definition).to eql({
       'type' => 'object',
-      'properties' => {},
-      'required' => ['hidden_prop']
+      'properties' => {}
     })
   end