Migrate trailing newlines out of nodes with auto-inserted end tags

* If a node has its end-tag autoinserted, then it is safe to
  migrated trailing newlines out of it.  The serializer works best
  when trailing newlines are outside HTML nodes, and this is part
  of the series of fixes trying to get the DOM to conform to that.

  Should we make the serializer more robust and not rely on this
  assumption (ex: parsoid clients, VE)?

* Fixes the following snippet from: en:World of our own which itself
  has fewer RT errors
{|
|<small>
a</small>
|}

* No change in parser test results (TODO: add new tests).

Change-Id: I82e73c8527bfcd52d72cdd7bfbf71bed9b88852b