it should not persist blank strings, but it does

I found a bug when trying to create a slug with a blank field, mongoid-slug stores a blank string broking `to_param` method.
Check it out https://github.com/lucasrenan/mongoid-slug/blob/master/spec/mongoid/slug_spec.rb#L28 (this test is still failing).

So, could you give any advice how should I fix this bug? I tried to include a condition for `build_slug` method. however, it wasn't a good idea.

Thanks.