  components has a
            "header" facet render a "tr" element. For each
            <code>UIColumn</code> that actually has a "header"
            facet, render
            it inside of a "th" element. Columns that don't have a "header"
            facet cause an empty "th" element to be rendered. Output the
            value of the "headerClass" attribute of the <code>UIColumn</code>
            component, if present, as the value of the "class" attribute on
            the "th". If the "headerClass"