Ensure that classes only get registered once

I have an odd issue in my project where descendent classes are getting registered with searchkick more than once (even though models declarations are correct). This leads to rake searchkick:reindex:all executing each model reindex twice. I'm still working to track down where exactly this is happening in my code, but the extend callback here probably should refuse to re-register classes anyway, so thought I'd send this simple PR over.