allow add of keys with array

This will allow adding multiple keys using array (http://www.codeigniter.com/user_guide/database/forge.html#adding-keys). Only if user wants, he can use the table columns to set a primary key by setting second parameter as TRUE.
diff --git a/system/database/DB_forge.php b/system/database/DB_forge.php
index d99fd00..865498f 100644
--- a/system/database/DB_forge.php
+++ b/system/database/DB_forge.php
@@ -239,7 +239,7 @@
 	 */
 	public function add_key($key, $primary = FALSE)
 	{
-		if ($primary === TRUE && is_array($key))
+		if (is_array($key))
 		{
 			foreach ($key as $one)
 			{