Hiding/showing columns breaks sort arrows

[Jacob Weber]

1. Load the "mvc" example.
2. Click the first "Grid" tab.
3. Click one of the headers, and the sort arrow will appear.
4. Run this in the console:

var grid = Ext.ComponentQuery.query('grid-grid')[0];
grid.hideColumn(0);
grid.showColumn(0);

1. The sort arrows are now gone. Even if you click the headers, they won't reappear.

I think this is because the column element is cached, but the header is re-drawn with new elements.