[Fix] Increase/decrease hash_key value for tokens depending on is_unlearn

[Fix] Increase/decrease hash_key value for tokens depending on is_unlearn

When relearning a message, this script is called twice:

1. With `is_unlearn` true and the old `is_spam` value
2. With `is_unlearn` false and the new `is_spam` value

If `is_unlearn` is true, the code should not increase the value of the hash_key. Decreasing it ensures that it behaves like a fresh learn with the new is_spam value.