Plugin `batch `does not work when enabled to nested serializer only

[Andrey Glushkov]

NoMethodError: undefined method get for nil:NilClass in remember_key_for_batch_loading method

https://github.com/aglushkov/serega/blob/master/lib/serega/plugins/batch/batch.rb#L270-L274

[Andrey Glushkov]

We must add batch plugin to parent serializer that has nested serializer with :batch attribute. Now we show meaningful error message instead of NoMethodError