Applying fix for issue Double sort indicators #453.

Issue is caused by Backgrid.Body.sort handling server-side PageableCollection differently than other collections.
Call to collection.sort() cannot be performed on a server-side collection, but Backgrid.Header relies on "sort" event to trigger resetting column directions.

This fix triggers sort event on server-side PageableCollections so that proper column sort resetting behavior is maintained.

Signed-off-by: Michael Russo <merusso@gmail.com>