(Bug 51721) Suppress indent-pre nowikis in ref content

* Indent-pres are suppressed in ref-content.  So, no reason to
  add nowikis around leading whitespaces when serializing ref
  tags either.
* One more html2html test now green.

Change-Id: I12d88c7cc59b95c0be884dd8fa3aa390aed122e8