`getForeignKeys()` - inconsistency of array keys

SQLite:

```php
[['auth_item', 'child' => 'name'], ['auth_item', 'parent' => 'name']];
```

MySQL:

```php
['auth_item_child_ibfk_1' => ['auth_item', 'child' => 'name'], 'auth_item_child_ibfk_2' => ['auth_item', 'parent' => 'name']];
```
