Fixed a bug in DB Forge, when inserting an id field (#3456).
diff --git a/system/database/DB_forge.php b/system/database/DB_forge.php
index 5f25ebe..2ddb21b 100644
--- a/system/database/DB_forge.php
+++ b/system/database/DB_forge.php
@@ -132,12 +132,13 @@
 		{

 			if ($field == 'id')

 			{

-				$this->fields[] = array('id' => array(

-										'type' => 'INT',

-										'constraint' => 9,

-										'auto_increment' => TRUE

-										)

-									);									

+				$this->add_field(array(

+										'id' => array(

+													'type' => 'INT',

+													'constraint' => 9,

+													'auto_increment' => TRUE

+													)

+								));

 				$this->add_key('id', TRUE);

 			}

 			else