Handle "!!|" variant of th-tag wikitext syntax

* The following wikitext wasn't RTing properly
{|
!| a !!| b
|}

* Fixed the startTagSrc recorder to detect this use case and
  update the startTagSrc for the th-tag.

* Fixes RTing of this snippet and eliminates similar RT errors
  from en:Walls Historic Site

* No change in parser tests (TODO: Add more)

Change-Id: If8b436d18f23e56ba2336d8cf3e4f6f45923dd5b