Set recursion available flag on nameserver responses

nameserver/response.go

@@ -27,6 +27,8 @@ func sendResponse(
 		// Dropping query.
 		return nil
 	}
+	// Signify that we are a recursive resolver.
+	reply.RecursionAvailable = true
 
 	// Add extra RRs through a custom RRProvider.
 	for _, rrProvider := range rrProviders {