Ensure relation.order method receives Arel to silence deprecation warning

[Polidoro]

Ransack v2.4.1 introduced the postgres_fields_sort_option configuration option (an awesome feature by the way!)

That change has the side effect of converting scope_or_sort from an Arel to a string. This triggers certain versions of Rails to fire a warning every time an order is applied if postgres_fields_sort_option is set.

This fix ensures that if the ordering is converted to a string when the NULLS FIRST/NULLS LAST is appended it is then converted back into an Arel before being passed to relation.order. This silences the warnings because the sql is no longer a string when it is run.

More info on the warning can be found here: rails/rails#32995

[Polidoro]

I need to double check that this is reproducible, so closing this until I do.