Update _item.php

Othervise if message time is not set it is shown in debug panel as

```
<span class="not-set">(не задано)</span>
```

(html entities are encoded prior to be displayed)


![aaa](https://cloud.githubusercontent.com/assets/2150916/3350274/d8482942-f99e-11e3-833d-d63dafee5ae6.png)

Better use

```php
        [
            'attribute' => 'time',
            'format' => 'datetime',
        ],
```