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

lib/active_model/serializer.rb

@@ -56,7 +56,7 @@ end
       attr_reader :key_format
 
       def serializer_for(resource, options = {})
-        if resource.respond_to?(:to_ary)
+        if resource.respond_to?(:each)
           if Object.constants.include?(:ArraySerializer)
             ::ArraySerializer
           else