natsmsg: multi-value header extraction; natsmsgtest: observable Nak

HeaderCarrier gains Values, satisfying propagation.ValuesGetter — without it the baggage propagator falls back to Get and reads only the first of multiple same-named headers, truncating multi-header W3C baggage on extract. FakeMsg.Nak now counts into a Naks field so a test can tell an undelayed Nak from no disposition at all.