Bug #52861 unset failes with ArrayObject and deep arrays

ArrayObject fails to unset multi-dimensional arrays.  This is due to the code that fools ZE to believe we have a reference check.  In this case it is missing BP_VAR_UNSET whereas BP_VAR_W and BP_VAR_RW are handled.  Changing this resolves the overall issue.  This does not cause a break in BC other than the obvious fixing of the bug. Thanks to @auroraeosrose for helping me locate and did it!

Example:
```php
$arrayObject(array('foo' => array('bar' => array('baz' => 'boo'))));
unset($arrayObject['foo']['bar']['baz']);
// will not work: PHP Notice:  Indirect modification of overloaded element of ArrayObject.
```

The patch resolves this behavior which has been broken since 5.3.4.