Fixed odd breakage of templates involving '<span class="{{foo}}">{{bar}}</span>'...

Fixed odd breakage of templates involving '<span class="{{foo}}">{{bar}}</span>'. {{bar}} was expanded without any arguments passed from the parent page. The problem was $this->mAssocArgs being overwritten by the call to replaceVariables() from within Sanitizer::removeHTMLtags(), and thus subtly corrupted for the subsequent call from braceSubstitution() itself. I've forgotten why I made that a class member in the first place, but making it a local instead doesn't break any parser tests, so it's a sufficient solution as far as I'm concerned.