dedup function issue with string IDs

[boynux]

I'm working to integrate Google tasks with org-sync and since gtasks IDs are strings and not numbers the dedup function org-sync-buglist-dups cannot detect duplicates.

The potential fix is to define the has table with a different equality test like:

(hash (make-hash-table :test `equal))

If that's not breaking anything else I can make PR for this change