Reset "down" flag if successful request is made

If a resource is flagged as "down" but a request is still made (see below for how that might happen), and that request was successful, the appropriate "down" flags should be removed.

If the `attempt_uncached_request_when_down` option is used, then an uncached request will still be sent even though the resource, regardless of scope, is recorded as being down. In that scenario, a successful request could remove the "down" flag.

If the `scope_for_availability_check` option is set to anything more specific than "host", a request may be made to a resource even though some component of it is recorded as being "down". For instance, a request could be made to `https://example.com/hello/` due to this option, even though a flag is set indicating that the host `example.com` is down. If that request were successful, the flag for `example.com` could be removed.