(Bug 48570) Fix subtle selser bug handling separator-only nodes

* When the original source for an element node whose content
  was entirely made up of separator text, selser duplicated the
  separator in leading and trailing separators. This manifested
  in the various bug reports around an extra space before links
  and colons (which introduces an &nbsp; <span> which exhibits
  this separator-only node behavior and causes the single-space
  duplication reported in the bug reports).

* This patch also fixes 11 additional selser tests.

Change-Id: Ibbc97e2eeb93fff56beb3fdc9cb2875ceb3e100b
Merge "(Bug 48570) Fix subtle selser bug handling separator-only nodes"