Don't negative cache 400 responses.

[Alan Carroll]

We are having this problem inside Yahoo! and it seems to me to be wrong to 
negative caches a 400 (HTTP_STATUS_BAD_REQUEST) because that is a user agent / 
proxy problem, not an origin server problem. It can lead to easy (if temporary) 
denial of service by simply sending a bogus request through ATS. I'd like to 
clip that status out of is_negative_caching_appropriate.