[Proposal] Fix for recent fillableFromArray commit 03f8744b

Given:
```php
$fillable = [ 'foo' ];
$unguarded = true; // perhaps after Eloquent::unguard()
$attributes = [ 'foo' => true, 'bar' => true ];
```

Broken behavior in 03f8744b
```php
CustomModel::create($attributes); // resulting in [ 'foo' => true ]
```

Workaround for broken behavior in 03f8744b
```php
$model = new CustomModel;
$model->fillable(array());
$model->fill($attributes);
$model->save(); // resulting in [ 'foo' => true, 'bar' => true ]
```

After this patch:
```php
CustomModel::create($attributes); // resulting in [ 'foo' => true, 'bar' => true ]
```
And it still works like 03f8744b when `$unguarded = false`.