Fixes to wikitext escaping in <th> tags

* "foo||bar" in the HTML below should be nowiki-escaped
  during serialization.  This patch fixes this.
-------------
<table>
<tr><th>foo||bar
</th></tr></table>
-------------

* 1 html2html test now passes, but 1 html2wt test now fails
  The html2wt failure is due to stale parser tests.  Once the
  parserTests.txt patch submitted for review is merged, some of
  these tests will get fixed.

Change-Id: I04863d63d8cbd36460fa6157944198ee680a97d5
Merge "Fixes to wikitext escaping in <th> tags"