header cell to render a span instead of an anchor when not sortable

If a column has `sortable` set to false, then it'd be nice if the headerCell didn't render an anchor tag.
The anchor tag often has default styling, looking like a link, which could mislead the user into thinking it is a clickable/sortable header. 

Attached is a pull request that uses a `span` tag instead of an `a` when `sortable: false`.