Fix bad pdo_options merge

We need this e.g. to be able to open a sqlite connection with
ATTR_PERSISTENT = true. array_merge was re-keying the array and
using the wrong attributes.

Change-Id: I1d272377b5142785ab17a91d7664d121241a565f

Author: ejegg

src/PHPQueue/Backend/PDO.php

@@ -39,7 +39,8 @@ public function __construct($options=array())
             $this->db_table = $options['db_table'];
         }
         if (!empty($options['pdo_options']) && is_array($options['pdo_options'])) {
-            $this->pdo_options = array_merge($this->pdo_options, $options['pdo_options']);
+        	// Use + operator instead of array_merge to preserve integer keys
+            $this->pdo_options = $options['pdo_options'] + $this->pdo_options;
         }
     }