API: Fixing issue mentioned on bug 16129 comment #3: for certain inputs, action=parse...

API: Fixing issue mentioned on bug 16129 comment #3: for certain inputs, action=parse dies with an internal error whining about $wgTitle not being set in OutputPage::parse(). This commit works around this by setting $wgTitle, but the parser and its friends should really leave $wgTitle alone, because people might want to parse non-$wgTitle titles, which the Parser::parse() docs suggest is possible (and it should be), but apparently isn't.