Bug fix serializing mw:Entity nodes.

* Fixes problem with mw:DiffMarker ending up in serialized
  output when the diff-marker is added by dom-diff inside the
  mw:Entity span.

Change-Id: Ib39c2e80633ba1aeab0ecb5c3c3c85f3abe3bb59
Merge "Bug fix serializing mw:Entity nodes."