Allow cancel ETA

[awarecan]

The Nest API gives a way to cancel an ETA. See link

This requires setting estimated_arrival_window_begin to int(0). However we don't allow that by limit estimated_arrival_window_begin to isoformat string.

python-nest/nest/nest.py

Lines 1552 to 1554 in 4222c4e

	data = {'trip_id': trip_id,
	'estimated_arrival_window_begin': eta_begin.isoformat(),
	'estimated_arrival_window_end': eta_end.isoformat()}

[awarecan]

I won't have time to work on it, PR is welcome.