Add seed values support for Query Builder order_by

(feature request #1987)
diff --git a/system/database/DB_driver.php b/system/database/DB_driver.php
index dfaf0e0..4fcb6c6 100644
--- a/system/database/DB_driver.php
+++ b/system/database/DB_driver.php
@@ -329,9 +329,9 @@
 	/**
 	 * ORDER BY random keyword
 	 *
-	 * @var	string
+	 * @var	array
 	 */
-	protected $_random_keyword = ' RAND()';
+	protected $_random_keyword = array('RAND()', 'RAND(%d)');
 
 	/**
 	 * COUNT string