Always update counter caches in memory when adding records

Before, calling `size` would only work if it skipped the cache, and
would return a different result from the cache, but only if:

- The association was previously loaded
- Or you called size previously
- But only if the size was 0 when you called it

This ensures that the counter is appropriately updated in memory.
Merge pull request #15747 from sgrif/sg-trolololol-this-is-so-broken

Always update counter caches in memory when adding records