FIX #355 Sorting in pageinated lists

If a many_many relation with the sort in an many_many_extraField has an item draged to another page the resulting sort
is wrong. This was caused by the handleMoveToPage function not respecting the original sort of the list.

src/GridFieldOrderableRows.php

@@ -500,7 +500,7 @@ public function handleMoveToPage(GridField $grid, $request)
         $move  = $request->postVar('move');
         $field = $this->getSortField();
 
-        $list  = $grid->getList();
+        $list  = $grid->getList()->sort($field);
         $manip = $grid->getManipulatedList();
 
         $existing = $manip->map('ID', $field)->toArray();