Adding Failing test for incremental ids

Issue - when saving post 2 more than once it should not be updating that slug to a-post-title-2 but it is (where as the behaviour for post one is working as expected)

@cviebrock It's for this issue here https://github.com/cviebrock/eloquent-sluggable/issues/59#issuecomment-59854928

I think this looks about right to replicate the issue