[ticket/15320] Redis cache does not save keys withouth expiration

In some functions like sql_save in cache/memory.php, code try to save a key/value in cache with ttl = 0 so key should never expire. In current redis.php cache driver, it fails so key never get cached. This cause for example that when you create a subforum, it not appear in the forum box in the admincp.
To solve, if ttl is 0, we use redis->set instead of setex

phpBB/phpbb/cache/driver/redis.php

@@ -137,6 +137,10 @@ class redis extends \phpbb\cache\driver\memory
 	*/
 	function _write($var, $data, $ttl = 2592000)
 	{
+		if($ttl == 0)
+		{
+			return $this->redis->set($var, $data);
+		}
 		return $this->redis->setex($var, $ttl, $data);
 	}