non-default formats cause 500 errors

[peff]

I noticed in the heroku logs that we generated an HTTP 500 for a request to /blog.rss. It's understandable, in that there is no RSS feed there any more. But probably we ought to be showing a 404 instead.

That one was presumably requested because it used to do something, but of course this extends to .json, etc. Is there a good way that we should be telling Rails that we don't have those other formats, and it should just generate a 404?