Fix broken HTML-pre serialization

* Got rid of HTML-pre path from the pre-handler and migrated
  that code into the generic HTML handler to deal with <pre>s
  and the lost leading newline problem.

* Fixes wt2wt (and relatd selser) failures from previous
  parserTests patch.

Change-Id: Ib575645f37bfc27567be98f1d802294175d1f064