Improve accuracy of makeSeparator in selser mode.

* makeSeparator was not skipping past non-content nodes
  while examining constraints.
* Improves html2wt & html2html output of previously failing tests.
* 14 additional selser tests passing; 1 selser test newly failing.

Change-Id: If3c06bc71f91774f00da9f4dc5fdd8e371c39004