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

Fixes #6812 
changing to iterator_to_array() make error in test : 
<code>
1) ZendTest\Paginator\PaginatorTest::testAcceptsTraversableInstanceFromAdapter
Failed asserting that ArrayIterator Object (...) is an instance of class "ArrayObject"
</code>