Fixed unset of Php::Value

The old unset can't work on associative array. because the key size is wrong.

for example:
```c++
void testUnset(Php::Parameters &params) {
    params[0].unset("hello");
}
```

```php
<?php
   $a = array("hello" => "world");
   var_dump($a);
   testUnset($a);
   var_dump($a);
?>
```

Expected output is:
```
array(1) {
  ["hello"]=>
  string(5) "world"
}
array(0) {
}
```

The actual output is:
```
array(1) {
  ["hello"]=>
  string(5) "world"
}
array(1) {
  ["hello"]=>
  string(5) "world"
}
```

This patch fixed it.