Added behavior for results_per_page parameter in request.

At the moment I can see only one unwanted (not sure about that) behavior in DataSource after this update. ``datasource_results_per_page`` parameter is always present in request and if you use ``getParameters`` at ``DataSourceView`` you will get an array with ``datasource_results_per_page`` unless its ``null``.

This PR is related to https://github.com/fsi-open/datasource-bundle/pull/5