adds method thenDoNothing() like in Mockito which helps mocking methods ...

...who don't return any value


I have a test case where I need to mock the following (short) example:

```php
namespace My;

class Response
{
    public function send()
    {
        $this->sendHeader();
        echo $this->body;
    }
}
```

With Mockito for Java, I would use the method thenDoNothing() which I missed here, so I have implemented it.

In this special case, PHPUnit does not alow you to send header information because it uses output buffering. That's why I need the workaround with thenDoNothing(). With PHPUnit, you can achieve the same thing as follows:

```php
$response = $this->getMock('\My\Response', array('send'));
```