Added serializer for authentication.resetAllPasswords

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

- resetAllPasswords was missing a serializer
- added a new passthrough serializer for now
- the tests already cover this endpoint

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

@@ -47,6 +47,10 @@ module.exports = {
         };
     },
 
+    resetAllPasswords(data, apiConfig, frame) {
+        frame.response = data;
+    },
+
     acceptInvitation(data, apiConfig, frame) {
         debug('acceptInvitation');