(Bug 50224) <source> is considered an ext-tag => it isn't a void elt

* Because of Bug 47579, 1467925e stopped tokenizing <source> as
  a HTML element, but left behind the check for <source> as a
  void (self-closing) element.

* This broke the tokenizing of <source> ... </source> extension
  use as reported in the bug.

* Bug fixed. No change in parser tests.

Change-Id: I0de1e49e2fdff76874ca48f88d8b8bb4ff75e923