Send nil when rescan is finished.

I previously fixed the duplicate send (before seeing GH issue #54),
but forgot that btcwallet expects a nil reply when rescan has
finished.  This adds the final reply back, but replies with nil.

Fixes #54.

rpcserver.go

@@ -1298,6 +1298,10 @@ func handleRescan(s *rpcServer, cmd btcjson.Cmd,
 		}
 	}
 
+	reply.Result = nil
+	mreply, _ := json.Marshal(reply)
+	walletNotification <- mreply
+
 	rpcsLog.Debug("Finished rescan")
 	return nil
 }