`until` should always return an Interval

[ab-pm]

When .until(…) is called on an invalid Datetime instance, it returns the instance itself instead of an invalid Interval.

To Reproduce

Interval.isInterval(DateTime.now().until(DateTime.invalid('demo'))) // true (ok)
Interval.isInterval(DateTime.invalid('demo').until(DateTime.now())) // false!
DateTime.isDateTime(DateTime.invalid('demo').until(DateTime.now())) // true!

It seems the bug was introduced in https://github.com/moment/luxon/commit/a2602aa4a315515a848036ec1c67268602b82a1d#diff-5146f40a8c8111b14e5ab3407eff80c7a6d2f3c92a4bf4ccd4647bff32d47e38R1317-R1319 (although even back then, it did sometimes return an interval and sometimes a Duration.invalid()). In diff() and .diffNow() this has been subsequently fixed during a reimplementation, but until still exhibits the unexpected behaviour.