Add seed values support for Query Builder order_by

(feature request #1987)
diff --git a/system/database/drivers/mssql/mssql_driver.php b/system/database/drivers/mssql/mssql_driver.php
index a9d53fb..53f2e55 100644
--- a/system/database/drivers/mssql/mssql_driver.php
+++ b/system/database/drivers/mssql/mssql_driver.php
@@ -53,9 +53,9 @@
 	/**
 	 * ORDER BY random keyword
 	 *
-	 * @var	string
+	 * @var	array
 	 */
-	protected $_random_keyword = ' NEWID()';
+	protected $_random_keyword = array('NEWID()', 'RAND(%d)');
 
 	/**
 	 * Quoted identifier flag