Fixes #6812 : change Paginator::getItemsByPage($pageNumber) handle AbstractResultSet

Fixes #6812 
`DbSelect::getItems()` never returned an array but always returned a `ResultSet`. The docs were wrong from the beginning.

See the current master: [`Zend\Paginator\Adapter\DbSelect` line 90](https://github.com/zendframework/zf2/blob/master/library/Zend/Paginator/Adapter/DbSelect.php#L90)