Fix a problem with generating a constructor when it is first time added.

Simplest way to reproduce:

```php

<?php

namespace spec\Acme;

use PhpSpec\ObjectBehavior;
use Prophecy\Argument;

class TestConstructorSpec extends ObjectBehavior
{
    function let(\stdClass $dependency)
    {
        $this->beConstructedWith($dependency);
    }

    function it_is_initializable()
    {
        $this->shouldHaveType('Acme\TestConstructor');
    }
}
```