php notice on "attribute_is_dirty" call if attribute is not dirty

If we pass a non-dirty attribute to `attribute_is_dirty($attribute)`, it raises a php notice.

Filename: lib/Model.php
Line Number: 558
```php
return $this->__dirty && $this->__dirty[$attribute] && array_key_exists($attribute, $this->attributes);
```

fix:
```php
return $this->__dirty && array_key_exists($attribute, $this->__dirty) && array_key_exists($attribute, $this->attributes);
```