Using only arrays in rules instead comma-separated string

I think the short form of array is not difficult to write than a comma separated string. But we gain a little bit in performance when creating each model instance.

```php
public function rules()
{
	return [
		[['name', 'email', 'subject', 'body'], 'required'],
	];
}
```
vs

```php
public function rules()
{
	return [
		['name, email, subject, body', 'required'],
	];
}
```