use userId instead of digging into a potentially undefined user hash

Ran into this issue with liquid.  If `use_auth` is falsy, a user object does not get added to the context that eventually gets applied to methods using `RESTstop.call()` or `RESTstop.apply()`.

Meteor method calls made via these two methods silently fail when trying to assign the userId property to the anonymous object passed into `new MethodInvocation` and the Meteor method never ends up being invoked.

THE FIX: `userId` already gets safely set above, so if we just use that instead, everything works out.