plugin/file: fix local CNAME lookup (#1866)

* plugin/file: fix local CNAME lookup

Issue #1864 explains it will, when we serve the child zone as well we
should just recursive into ourself (upstream self). Thus relax the
IsSubDomain check in file/lookup.go and just query (even if the query
will hit a remote server).

I've looped over all other plugins that do something similar (CNAME
resolving) and they didn't do the IsSubDomain check; therefor I've
removed it from *file* as well.

Added test in file_upstream_test that shows this failed before but now
results in a reply.

Fixes #1864

* self does not need to be exported

* Fix test

We don't know if we had a valid reply. Check this.

plugin/backend_lookup.go

@@ -122,12 +122,9 @@ func AAAA(b ServiceBackend, zone string, state request.Request, previousRecords
 				}
 				continue
 			}
+
 			// This means we can not complete the CNAME, try to look else where.
 			target := newRecord.Target
-			if dns.IsSubDomain(zone, target) {
-				// We should already have found it
-				continue
-			}
 			m1, e1 := b.Lookup(state, target, state.QType())
 			if e1 != nil {
 				continue