How do I mock a protected property of a an object.

Given
```
Class A {
protected $x = false;

public function methodA()
{
if($x){ return 'FOO'; }
return 'BAR';
}
}
```

My test is -
```
$a = new A();

\Aspectmock\Test::double($a,['x' => true]);

$this->assertEquals('BAR', $a->methodA() );
```

I get `null`.

How do I solve this?