Make sure `render json: ..., each_serializer: ...` is working with Enumerables

If you try to render Enumerable with custom `each_serializer` then it will render with default serializer instead:

```ruby
render json: sequel_scope, each_serializer: CustomSerializer
```

This commit fixes this behaviour. Most likely fixes https://github.com/rails-api/active_model_serializers/issues/664 as well
Merge pull request #665 from andreychernih/fix-each-serializer-option

Make sure `render json: ..., each_serializer: ...` is working with Enumerables