Close listener after calling StartRoutine()

This change prevents the listener from closing itself before
establishing the websocket session with the client.

app/client_context.go

@@ -62,6 +62,12 @@ func (context *clientContext) goHandleClient() {
 	}()
 
 	context.app.server.StartRoutine()
+
+	if context.app.options.Once {
+		log.Printf("Last client accepted, closing the listener.")
+		context.app.server.Close()
+	}
+
 	go func() {
 		defer context.app.server.FinishRoutine()