Readable content negotiation

While tracing through the format negotiation code, I found it helpful to make a couple of readability improvements to the big regular expression used in Accept-header parsing.

While I was there, I also took out what seemed to be a no-operation replacement (though I suspect there was *some* reason for it going in there in the first place, I couldn't think why).

They're separate commits, in case I'm wrong about the no-op :-)