Prevent mw:Entity spans from stopping dsr propagation.

* Fixes RTing of WS & comments in snippets like "&nbsp;  &nbsp;"
  and any other code with intervening entities.

* 1 more wt2wt, and 6 more selser tests green.

* Fixes WS RT diffs for en:Fortran 90, en:MoS:HEAD

* Unrelated fixes:
  1. Emit separator metas only if there is something
     to emit.
  2. Addressed old gerrit comment about separator verification
     regexp.

Change-Id: I13307e07f1baa5630a0836bdfd3dc026a789a7aa