jquery.tablesorter: Only look at th's for headers

If there would be td cells inside the thead, then these would be
counted too for longest header row. This was not possible in wikitext,
but was possible in MediaWiki tables.
And $.unique sets fields to undefined instead of deleting them, which
causes a nasty tough to spot counting mistake (which was even known
due to a testcase having a comment confirming that the count was off)

Bug: 53527
Change-Id: Ie551cd62275dd80560643131d68435166732d367
Merge "jquery.tablesorter: Only look at th's for headers"