Activity Getfields does not return source_contact_id

When using getfields with the Activity entity in the API an incomplete list of fields is returned.At present it only returns fields present in the civicrm_activity database table. However source_contact_id is a required field (for the REST API at least). target_contact_id is also a valid a field.

This has the side effect of making it seem like the API can/does not update the civicrm_activity_contact, where as it fact it can.