Description:
------------
Can't remove item when using unset() with a Yaf_Config_Simple instance

Test script:
---------------
<?php

$config = new Yaf_Config_Simple(array(
    'foo' => 'bar',
), false);

unset($config['foo']);
print_r($config);

Expected result:
----------------
Yaf_Config_Simple Object
(
    [_config:protected] => Array
        (
        )

    [_readonly:protected] => 
)

Actual result:
--------------
Yaf_Config_Simple Object
(
    [_config:protected] => Array
        (
            [foo] => bar
        )

    [_readonly:protected] => 
)