Bug: 70370 Explicitly specify content model in PHPParseRequest

This prevents mediawiki from choosing an alternate content model when
we know we are passing wikitext. This is causing math rendering to
fail in Flow due to the content model being detect as 'flow-board' but
the content sent is wikitext.

Change-Id: I703737a652284e751e5328f0aa2955fe51ae953a