Allow th tags in tbody

[skipchris]

Previously, a th tag inside a tbody > tr would cause issues with sort.

This is bad! We want to encourage <th scope="row"> in our markup. :)

This 1-line commit fixes that, and updates tests to match.

[peterwake]

@skipchris thanks for this .. it was useful. Especially when using "sticky" table headers.. ref for example https://css-tricks.com/a-table-with-both-a-sticky-header-and-a-sticky-first-column/