no population returned for some isochrones

[TheGreatRefrigerator]

the api returns a 3099 error Unable to compute isochrone attributes. for the following request
https://api.openrouteservice.org/isochrones?api_key=my-key&attributes=total_pop&format=json&interval=60&location_type=start&locations=8.411751,49.352414&profile=driving-car&range=540&range_type=time

with interval=540 it works
also with range up to 300 and interval=60
but range=360 then fails
but range=360 and interval=360 works again

[TimMcCauley]

Same as #297, fix in PR #330