Make sure Parsoid doesn't get snobbish and treat non-html5 tags badly.

* A recent patch to handle extension tags broke handling of html4
  tags like <big>, <center>, etc.
* The new test make sure we catch such regressions before going to
  RT testing.

Change-Id: Ia1f9247d2172d2d4c8d39cce838f896cf9e81fd7
Merge "Make sure Parsoid doesn't get snobbish and treat non-html5 tags badly."