Validate `attributes`  in top-level member
Validate `attributes`  in top-level member

According to the [JSON API specification](http://jsonapi.org/format/#document-resource-objects), a resource object **MAY** contain an `attributes` top-level member.

This change brings redux-object closer to the spec by first checking the presence of `attributes` instead of assuming it.
yury-dymov/redux-object