eliminate ActiveRecord as a dependency for errors

[SamMarkGoldman]

if ActiveRecord is not loaded in your project, but the method jsonapi_render_errors is called, the code tries to see if the object passed in is an active record object or not. But in doing so, it assumed active record is loaded to make said comparison.

If active record is not loaded, now the active_record_obj? method in JSONAPI::Utils::Response::Formatters always fails, which is good.

[tiagopog]

Although It's an unusual use case it's a good idea to make this check you've included. Thanks for contributing 👍