Fix ResponderRequest unmarshalling of IncidentResponders

[allyjweir]

This PR fixes an unmarshalling bug that was found while trying to use the ResponderRequestWithContext method on the client.

The response that the API returns is subtly different to the one which the client library is expecting. Within the responder_request_target object, the client library expects a key called incident_responders. However when interacting with the API directly, this key is actually incidents_responders. Note the additional s which pluralises 'incident'.

Impact

This meant that whenever you made a responder request via the client library, it would succeed but the response would only be partial. That meant you could not retrieve any of the information about the responders which your request had added to an incident.

A note on API documentation

Please note that there is an inconsistency in the PagerDuty API documentation for this endpoint.

• In the written documentation it refers to: incident_responders
• In the 'response example', it correctly refers to: incidents_responders

[ChuckCrawford]

Thanks for submitting this @allyjweir. Just following up on something internally and then we will get this merged.