Switch the 'unreachable' url from http://asdf.asdf to http://0.0.0.0

Some ISPs / proxies serve a valid HTTP response on any DNS miss, but
we are trying to test an unreachable server.
Merge pull request #2186 from justinsb/avoid_dns_catchall

Switch the 'unreachable' url from http://asdf.asdf to http://0.0.0.0