Dont normalize WS in mw:Entity span nodes.

- Fixes RTing of "a &nbsp; b" with --oldtext option and eliminates
  some RT errors from wp pages like en:Hospet
- Got rid of the installCollector function in serializer and
  directly constructed the 'endTagMatchTokenCollector' where
  needed.
- Fixed config error in parserTests.js from previous commit.
- Improved regexp in the migrateTrailingNLS dom post processor.

Change-Id: I4bb9409ab4473df85bd297a81659f62e955dd16c