No cache invalidation on save when slug changes

[diegobz]

Cache invalidation doesn't work when saving an entry with changes in the slug field.

[orcasgit]

After reviewing the code, it looks like we should invalidate the cache based on the slug prior to saving to catch this case (since the cache key is based on the slug). This would remove cache keys for items that changed their slugs and a new query for that slug will insert the new slug & key into the cache.