Added Phalcon\Mvc\View::__isset Magic method

isset function of the view will be false.

PHP Controller:

```php
class IndexController extends \Phalcon\Mvc\Controller
{
    public function indexAction()
    {
        $this->view->msg = 'Message';
    }
}
```

PHP View:

```
<?php echo $this->view->msg ?>
<?php if (isset($this->view->msg)): ?>
Enable Message
<?php endif ?>
<?php if (isset($this->view->test)): ?>
Enable Test
<?php endif ?>
```

Output:

```
Message
```

Update Output:

```
Message
Enable Message
```