(Bug 53229): Dont strip sole child if it is an element

* en:Jay_Bouwmeester had an autoinserted <td> with an empty
  <b></b> node inside and was being stripped by the dom cleanup
  and added back in which got fostered out and tripped up RT-ing.

* The previous patch was missing a check for !DU.isElt(node.firstChild)
  Added it in and that page now RTs cleanly.

Change-Id: Ic7b81bd99c95e31d251fbcf5fcea2320ea3082f0