EventsAPISession uses `X-Routing-Key` instead of setting `routing_key` in payload

[markafarrell]

EventsAPISession uses the X-Routing-Key header to specify the routing key and does not set the routing_key parameter in the REST Payload.

API specification requires that the routing_key parameter is a required parameter.

https://developer.pagerduty.com/docs/events-api-v2/trigger-events/