Fix error with setting object properties via Caller (previously Subject)

The following code will throw an error

```php

// ...

class SomeSpec extends ObjectBehavior
{
    function it_should_do_something()
    {
        $this->someProperty = 'value';
    }
}
```

The error is: 
> Argument 1 passed to PhpSpec\Wrapper\Unwrapper::unwrapAll() must be of the type array, integer given [...]
> called in [...]PhpSpec/Wrapper/Subject/Caller.php on line 73 and defined in [...]PhpSpec/Wrapper/Unwrapper.php line 10

Trying to set `$this->someProperty` in the spec goes via `Subject::__call` -> `Subject::setToWrappedObject` to `Caller::set`

Changing the call to from `Unwrapper::unwrapAll` to `Unwrapper::unwrapOne` during `Caller::set` fixes this issue.