app.enableAuth: correctly detect attached models

Fix a typo in "app.enableAuth" that caused the method to not detect
the situation when e.g. the built-in User model is already attached
to a datasource.

lib/application.js

@@ -327,8 +327,10 @@ app.enableAuth = function(options) {
           g.f('Authentication requires model %s to be defined.', m));
       }
 
-      if (m.dataSource || m.app) return;
+      if (Model.dataSource || Model.app) return;
 
+      // Find descendants of Model that are attached,
+      // for example "Customer" extending "User" model
       for (var name in appModels) {
         var candidate = appModels[name];
         var isSubclass = candidate.prototype instanceof Model;