fix(core): sort priorities were not displaying when 2nd sort was added

src/js/core/directives/ui-grid-header-cell.js

@@ -33,8 +33,8 @@
             };
             $scope.isSortPriorityVisible = function() {
               //show sort priority if column is sorted and there is at least one other sorted column
-              return $scope.col.sort.priority !== undefined && $scope.grid.columns.some(function(element, index){
-                  return element.sort.priority !== undefined && element !== $scope.col;
+              return angular.isNumber($scope.col.sort.priority) && $scope.grid.columns.some(function(element, index){
+                  return angular.isNumber(element.sort.priority) && element !== $scope.col;
                 });
             };
             $scope.getSortDirectionAriaLabel = function(){