JSON sent:
{"app_id":"45eb2d31-de93-4b77-a3ca-255d1c5d7e9f","included_segments":["All"],"data":{"foo":"bar"},"large_icon":"ic_launcher_round.png","contents":{"en":"Testing Message"}}

JSON received:
{"allresponses":"{\"errors\": [\"Access denied.  Please include an 'Authorization: ...' header with a valid API key (https:\/\/documentation.onesignal.com\/docs\/en\/keys-and-ids#api-keys).\"]}\n"}