Fix crasher introduced by cee6e738

The following page now parses without crashing:
en:Philippine_House_of_Representatives_elections_in_the_Zamboanga_Peninsula,_2013

Change-Id: Ieb530456ca87fdecce8a5f1f2dbf6be83528cd67
Tweaks to td-fixup code

1. The tokenizer never inserts a closing </td> tag.  So, non-HTML
   <td>s will always have their closing tag auto-inserted by
   the HTML treebuilder ==> auto-inserted-end check for <td> tags
   is not required.

2. The test requires a check of whether nextNode is a wrapper element
   of a transclusion. DU.isTplElementNode is meant to check for content
   produced by a template and only identifies top-level wrapper elements
   currently.  But, it could easily be changed in the future to identify
   all template content nodes. So, I used DU.isEncapsulatedElt instead.

   In a separate patch, it might be useful renaming and clarifying the
   semantics of these two seemingly similar functions.

Change-Id: Ibfcae0862055eeca0debbbe846e194d29577d1e1