Fix _per_page and _page management in datagrid

Find a bug when filtering results the test isset($this->values['_per_page']['value']) return true when '_per_page' => 25 (Using php 5.3.28)
This PR solve the problem
It would be nice if we check the existence of `$this->values['_page']['value']` so no notice will be produced if some invalid data are passed to datagrid. See #2313