Added serializer for members.destroy

refs: https://github.com/TryGhost/Toolbox/issues/245

- This was the only endpoint in the members API that didn't have a serializer
- It just needs to be a simple passthrough, but we define it so we can change the default

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

@@ -9,6 +9,8 @@ module.exports = {
     read: createSerializer('read', singleMember),
     edit: createSerializer('edit', singleMember),
     add: createSerializer('add', singleMember),
+    destroy: createSerializer('destroy', passthrough),
+
     editSubscription: createSerializer('editSubscription', singleMember),
     createSubscription: createSerializer('createSubscription', singleMember),
     bulkDestroy: createSerializer('bulkDestroy', passthrough),