Entity inherited class with custom attributes does not override mother c...

...lass attributes

Inhirited class should combine its attributes with the attributes of super class

```
class Book
  include Lotus::Entity
  self.attributes = :title, :author
end

class NonFictionBook < Book
  self.attributes = :age
end


NonFictionBook.attributes.must_equal [:age, :title, :author]
```

Notes: `Utils::ClassAttribute` is useful for this job as it does inheritance well