API: Allow special pages for action=parse&title=

Iec98e472 had broken this. Use the same strategy as in
MediaWiki::initializeArticle to work around the problem.

Bug: 49477
Change-Id: Ib272b1f9395f6adade505ac7c6a24f9a4bff2779
Merge "API: Allow special pages for action=parse&title="