fix bug in instantinate with some arguments

There is a bug in instantinate with some arguments.

Example:

```php
class SomeSpec extends ObjectBehavior
{
    public function let()
    {
        $this->beConstructedWith('foo');
    }

    public function it_is_foo()
    {
        $this->isFoo()->shouldBe(true);
    }

    public function it_is_bar()
    {
        $this->beConstructedWith('bar');
        $this->isBar()->shouldBe(true);
    }
}
```

`it_is_bar` spec is failed in this case.
Because, it is not done to reset the instance at `beConstructedWith`.