Fix DSR to compute dsr across mw:Placeholder spans.

* Helps fix DSR on wikitext like "a : b" which has a &nbsp;
  placeholder before the ":" because of a tokenizer production.
* The fixed DSR enables RTing of wikitext like:
  "*a\n\nb : c"
  Reduced test case found from: en:Telecommunications in Sudan
* Minor code cleanup.
* No change in parser tests.

Change-Id: I2d8338da6f80bb4329a686084c06a89ec7c26e3d