* Change 'on' to 'heat' as fallback thermostat mode
'on' isn't recognised as a mode by Google Assistant, rather is used as a method to return a thermostat to a previous mode. In the case where a thermostat doesn't support the standard google modes (e.g. A homematic radiator thermostat) this means the set_temperature doesn't get returned to the user on a request.
* Update test_google_assistant.py