middleware/proxy: healthchecks fixes (#183)

* middleware/proxy: add spray keyword

When spray is used, the proxy will, when all backend are down, spray to
each target. When not used, default to the old defaults: max 1 failure
and no spray. These defaults are also used when forwarding queries to
another CoreDNS instance.

Update the README with the new keyword.

* typos

* Make MaxFail = 1 again

* more reversals

middleware/proxy/upstream_test.go

@@ -10,6 +10,7 @@ func TestHealthCheck(t *testing.T) {
 		from:        "",
 		Hosts:       testPool(),
 		Policy:      &Random{},
+		Spray:       nil,
 		FailTimeout: 10 * time.Second,
 		MaxFails:    1,
 	}