I think the Shwarma thing was an ad lib. You actually see the Shwarma place right next to Iron man when he lands after killing one of the big worm monsters, and the post-credits scene was filmed a week before the movie premiered, long after post production was formally finished. RDJ probably saw the place while filming and ad libbed the line, and then Joss decided to get everyone together to film the restaurant scene at the last minute.