== Cancan should automatically protect updates just like it protects create

To reproduce:

* rake db:migrate
* rails server
* http://localhost:3000/posts
* Create a new post with a User ID of 5
** Note this is prohibited by cancan
* Create a new post with a nil User ID
** Note this is allowed by cancan (you aren't signed in, your user_id is nil)
* Edit your post, and set the User ID to 5
** Note this succeeds
* Edit app/controllers/posts_controller.rb on line 6, uncomment the before_filter line and save
* Create a new post with a nil User ID
** Edit the post and set the User ID to 5
** Note the problem is now fixed, guest users cannot create or update posts that don't belong to them.