Sluggable fix an exception with slug endding with 0XXX

Hi,

There is an error with the new sluggable optimization. If there is a slug ending with 0XX this can break the sluggable behavior

if you have those slug in your database :

* hello-world-1
* hello-world-000
* hello-world-2

if you try to insert a new `hello-world` title it will be inserted has hello-world-1 and get an SQL Exception `1062 Duplicate entry 'hello-world-1'`

This PR add a test to prove the issue, and fix it.