(Bug 53464): Improved detection of missing opening HTML tags

* Improved autoInsertedStart detection plus also added a check to
  mark such a tag as originating in HTML where it is possible to
  infer this. This fixes parse, DSR, non-selser as well as selser
  serialization of the test case in the bug.

* Added a Parsoid-only test case for this scenario (which passes)

* Two additional selser tests now seem to pass with the fix
  (possibly because DSR is more accurate now?). I didn't investigate
  the reasons.

Change-Id: I802922eaee67b157e417ece72e4ca7338ec25426