Spam can be unmarked

https://trello.com/c/OC71NU4i/27-as-an-admin-i-can-unmark-posts-as-spam

![selection_017](https://cloud.githubusercontent.com/assets/8248100/5019213/71b3b30a-6a8b-11e4-8f8a-3bddfe39f866.png)


No same thing as `post.update(spam: false)`