departures/arrivals: cache as atom if opt.duration missing

[derhuerst]

Currently, we incorrectly try to cache a departures()/arrivals() call as a collection, even if it has no opt.duration. In this case, duration is NaN.

cached-hafas-client/index.js

Lines 196 to 197 in a9e7813

	if (!('duration' in opt)) useCache = false
	const duration = opt.duration * MINUTE

A sophisticated solution would be to "re-compute" duration once the departures/arrivals have been fetched from HAFAS. But I think a simpler solution would be sufficient: If opt.duration is missing, cache the results as an atom.