Grid is lacking header classes when created from HTML table.

[martintabakov]

Bug Report

When creating the Grid from pre-defined HTML table, the header cells are missing classes. That results in inconsistency when using the Themebuilder to style the component.

Reproduction of the problem

1. Open this Dojo example, which contains a Themebuilder-generated stylesheet - https://dojo.telerik.com/wpmTrWPf/3

Current behaviour

The HTML for the first column is manually added to the predefined markup. The second column is left as it is. As a result, the second column is not being styled according to the stylesheet because it is missing HTML and classes

Expected behaviour

The Grid should render the column headers as if (or at least similar ) they are defined through the columns configuration.