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/lookup.go

@@ -1,6 +1,6 @@
 package proxy
 
-// function OTHER middleware might want to use to do lookup in the same
+// functions OTHER middleware might want to use to do lookup in the same
 // style as the proxy.
 
 import (
@@ -77,8 +77,8 @@ func (p Proxy) lookup(state middleware.State, r *dns.Msg) (*dns.Msg, error) {
 		// hosts until timeout (or until we get a nil host).
 		for time.Now().Sub(start) < tryDuration {
 			host := upstream.Select()
+
 			if host == nil {
-				// TODO(miek): if all HC fail, spray the targets.
 				return nil, errUnreachable
 			}