This ```first()``` only if ```current()``` call before ```rewind()```
In other cases ```current()``` is ```current()```.
This added because for this case:
```php
$pl = new PriorityList();
$pl->insert('n1', 'v1', 0);
$pl->insert('n2', 'v2', 100);
$current = $pl->current();
```
```$current``` should be 'v2'