Table._windowed_delta does not respect relative_to_field()

[cowboygneox]

https://github.com/faust-streaming/faust/blob/master/faust/tables/base.py#L564

It looks like when using the .delta(timedelta), the delta does not respect if the table was configured to use a different relative relationship, whether it be relative_to_now or relative_to_field. This makes delta useless if you need to configure your table with a different relative.

Is this by design or is this a bug?

Thanks!