Fixed date formatting in members CSV export

no issue

- Made date formatting coherent with the one used in API and the exporter
- Using JSON.stringify() here because that's exactly how API is getting it's formattting done atm

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

@@ -69,8 +69,8 @@ module.exports = {
                 subscribed_to_emails: member.subscribed,
                 complimentary_plan: member.comped,
                 stripe_customer_id: stripeCustomerId,
-                created_at: member.created_at,
-                deleted_at: member.deleted_at,
+                created_at: JSON.stringify(member.created_at),
+                deleted_at: JSON.stringify(member.deleted_at),
                 labels: labels
             };
         });