Fix dns-01-003 (#1634)

* plugin/{cache,forward,proxy}: don't allow responses that are bogus

Responses that are not matching what we've been querying for should be
dropped. They are converted into FormErrs by forward and proxy; as a 2nd
backstop cache will also not cache these.

* plug

* add explicit test

plugin/cache/setup.go

@@ -42,6 +42,7 @@ func setup(c *caddy.Controller) error {
 				x.MustRegister(cacheHits)
 				x.MustRegister(cacheMisses)
 				x.MustRegister(cachePrefetches)
+				x.MustRegister(cacheDrops)
 			}
 		})
 		return nil