Grid.sort doesn't reset previously sorted column's sort caret

[wyuenho]

Steps to reproduce:

1. grid.sort("col1", "descending");
2. grid.sort("col2", "ascending");
3. col1 's sort caret is still in descending order

Expected behavior:

col's sort caret should not be displayed and the column's direction shouldn't be null.

[ponimas]

Still broken with Backbone.PageableCollection in server-mode

[wyuenho]

@ponimas checkout #453 and see if you can reproduce this with a jsfiddle.