[RedisCache] Fix call to getPayload on int. The library is badly documented, but seems to return the TTL instead of the normal redis response

Issue description as reported by aab:

About one day after enabling redis plugin:

PHP Fatal error:  Uncaught Error: Call to a member function getPayload() on int in /var/www/html/plugins/RedisCache/RedisCachePlugin.php:96
Stack trace:
0 /var/www/html/lib/event.php(89): RedisCachePlugin->onStartCacheSet('gnusocial:gatea...', Object(Queue_item), NULL, 86400, false)
1 /var/www/html/lib/cache.php(202): Event::handle('StartCacheSet', Array)
2 /var/www/html/classes/Memcached_DataObject.php(496): Cache->set('gnusocial:gatea...', Object(Queue_item))
3 /var/www/html/classes/Memcached_DataObject.php(416): Memcached_DataObject->encache()
4 /var/www/html/classes/Managed_DataObject.php(620): Memcached_DataObject->update(Object(Queue_item))
5 /var/www/html/classes/Queue_item.php(74): Managed_DataObject->update(Object(Queue_item))
6 /var/www/html/lib/dbqueuemanager.php(75): Queue_item::top(Array, Array)
7 /var/www/html/lib/iomaster.php(287): DBQueueManager->poll()
8 /var/www/html/lib/iomaster.php(161): IoMaster->poll()
9 /var/www/html/scripts/queuedaemon.php(112): IoMaster->service()
10 /var/www/html/lib/spawni in /var/www/html/plugins/RedisCache/RedisCachePlugin.php on line 96

plugins/RedisCache/RedisCachePlugin.php

@@ -93,7 +93,7 @@ class RedisCachePlugin extends Plugin
             return true;
         }
 
-        if ($ret->getPayload() === "OK") {
+        if (is_int($ret) || $ret->getPayload() === "OK") {
             $success = true;
             return false;
         }