Fix array decoding polymorphic relationships

[Sauruz]

In polymorphic relationships the nested content is not decoded/casted to an array.

This fix will test again if the nested json object can be decoded to an array.

See: https://github.com/whitecube/nova-flexible-content/issues/435

[Keko-94]

@Sauruz Thank you for the fix but the validations doesn't work properly. Try to add required in your meta_title and check if it passes.