Upstream: honor the "down" flag for a single server.

Upstream: honor the "down" flag for a single server.

If an upstream block was defined with the only server marked as
"down", e.g.

    upstream u {
        server 127.0.0.1:8080 down;
    }

an attempt was made to contact the server despite the "down" flag.
It is believed that immediate 502 response is better in such a
case, and it's also consistent with what is currently done in case
of multiple servers all marked as "down".