### Bug report

When custom classes are set via the headerAttributes of the column and the showColumn is called, the predefined classes are wiped away

### Reproduction of the problem

1. Set custom class via headerAttributes.

2. Call the showColumn method.

3. Custom classes are wiped away.

Dojo sample for reproduction:

https://dojo.telerik.com/UJalevej

### Expected/desired behavior

The classes should be retained after the method is called.

### Environment

* **Kendo UI version:** 2021.1.119
* **jQuery version:** 1.12.4
* **Browser:** [all]