Non-column attributes

Having a resource like this:

```
class Api::V1::TripResource < Api::V1::BaseResource
  attribute :test

  def test
    123
  end
end
```

`$ rails g jsonapi:swagger Api::V1::Trip` fails with `jsonapi-swagger-0.7.1/lib/generators/jsonapi/swagger/templates/swagger.json.erb:80:in block (2 levels) in properties': undefined method []' for nil:NilClass (NoMethodError)`