[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.

lualib/redis_scripts/bayes_learn.lua

@@ -26,7 +26,7 @@ redis.call('HSET', prefix, 'version', '2') -- new schema
 redis.call('HINCRBY', prefix, learned_key, is_unlearn and -1 or 1) -- increase or decrease learned count
 
 for i, token in ipairs(input_tokens) do
-  redis.call('HINCRBY', token, hash_key, 1)
+  redis.call('HINCRBY', token, hash_key, is_unlearn and -1 or 1)
   if text_tokens then
     local tok1 = text_tokens[i * 2 - 1]
     local tok2 = text_tokens[i * 2]
@@ -41,4 +41,4 @@ for i, token in ipairs(input_tokens) do
       redis.call('ZINCRBY', prefix .. '_z', is_unlearn and -1 or 1, token)
     end
   end
-end
+end