Conform to ActiveModel::Serializers' way of determining array-ness

I ran into a problem with a model that uses friendly_id where the collection returned was a `FriendlyIdActiveRecordRelation`, which of course did not give the right answer when we asked it `is_a?(Array)`.

So I looked at how ActiveModel::Serializers does its determination of whether a resource is an array, and it calls `respond_to?(:to_ary)` (c.f. https://github.com/rails-api/active_model_serializers/blob/master/lib/active_model/serializer.rb#L274). So I just did the same.