Fixed labs.enabledMiddleware isSet reference (#13227)
Fixed labs.enabledMiddleware isSet reference (#13227)

no-issue

The `this` used by enabledMiddleware is not bound to `module.exports`.
This updates the middleware to reference the `isSet` method correctly
e10/Ghost,janvt/Ghost,TryGhost/Ghost,TryGhost/Ghost,ErisDS/Ghost,TryGhost/Ghost,ErisDS/Ghost,ErisDS/Ghost,e10/Ghost,janvt/Ghost