Additional tokenizer fixes for empty attrs in td-cells.

* The following wikitext was not RT-ing properly since
  startTagSrc wasn't being recorded properly for the td-cells.
---------
{|
| align="center" | foo ||  |
|}
---------

* Wikitext extracted from en:Spreuerbrücke which now RTs without
  errors.

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

Change-Id: I5a4f93b98ee062e53dc67d4d04cc6b4f447708c6