Output is not cleared after specify

Hi.

I've a test:

``` php
public function testSomething()
{
$this->specify(
'expect output',
function () {
$this->expectOutputString('lorem ipsum');
(new TestedClass)->doSomething();
}
);
$this->specify(
'expect another output',
function () {
$this->expectOutputString('lorem ipsum');
(new TestedClass)->doSomethingElse();
}
);
}
```

The first specification passes.
Second specification is failing. It expect output to be "lorem ipsum 2", but gots "lorem ipsum 1lorem ipsum 2".