Updated members serializer to handle POJO

no-issue

The @tryghost/members-api module is being updated to export a BREAD
service which will be used to move the logic from the controller into.
This service is currently designed to returns objects rather than
models, as it has to do manipulation of the returned data at the object
level. This update to the serializer will allow a seamless transition to
the use of the BREAD service and allow us to pull out the logic from the
controller sooner!

core/server/api/canary/utils/serializers/output/members.js

@@ -100,7 +100,7 @@ function exportCSV(page, _apiConfig, frame) {
  * @returns {SerializedMember}
  */
 function serializeMember(member, options) {
-    const json = member.toJSON(options);
+    const json = member.toJSON ? member.toJSON(options) : member;
 
     let comped = false;
     if (json.subscriptions) {