Include <th> elements in HTMLTableRowElement.cells

See:
* http://www.w3.org/TR/REC-html40/struct/tables.html#h-11.2.6
* http://www.w3.org/TR/REC-DOM-Level-1/level-one-html.html#ID-82915075
* http://www.w3.org/TR/html5/tabular-data.html#dom-tr-cells
* http://www.whatwg.org/specs/web-apps/current-work/multipage/tabular-data.html#dom-tr-cells