Fix/tweak to WTS separator extraction to be more robust.

* Commit 7ee4d090 tweaked p-wrapper output to make it more suitable
  for how WTS expects newlines.

* This patch tweaks WTS separator extraction code to be less
  insensitive to non-newline whitespace that shows up in between.

* With this patch, the following two html snippets serialize to
  the same wikitext.

  "<p>a\n </p>\n<p>b</p>" | node parse --html2wt ===> "a\n \nb"
  "<p>a</p>\n \n<p>b</p>" | node parse --html2wt ===> "a\n \nb"

* No change in parser test results, but fewer DSR inconsistency
  warnings in html2html runs.

Change-Id: I2bda392ebe95b7b31043ae7ec8a2e667f2e6d5d9
Merge "Fix/tweak to WTS separator extraction to be more robust."