Fixed Base\Object::toArray when using DateTimeImmutable from configuration

[mxcoder]

When using configuration like:

      dateTime:
          useDateTimeClass: true
          dateTimeClass: DateTimeImmutable

The base objects toArray method is unable to process the DateTimeImmutable, because its checking directly for \DateTime, the current patch makes it use DateTimeInterface instead.

[marcj]

Thanks, good job!