Nested groups does not work

```
SimpleRouter::group(['prefix' => 'v1'], function () {
    SimpleRouter::get('/', function () {
        echo "OK 1";
    });
    SimpleRouter::group(['prefix' => '/system'], function () {
        SimpleRouter::get('/', function () {
            echo "OK 2";
        });
    });
});
```

Route `/v1` works, but `/v1/system` does not.
