client request lock inside if network is closed or i/o time out

[gummy789j]

The following experiment would be lock at client.Request because network is closed or i/o time out, can you export a method to check the network is closed or not to avoid lock at client.Request.

	config := xrpl.ClientConfig{
		URL: "wss://s.altnet.rippletest.net:51233/",
	}
	client := xrpl.NewClient(config)
	err := client.Ping([]byte("PING"))
	if err != nil {
		panic(err)
	}

        // test send request after close
       r.Close()
	
	resp, err := client.Request(xrpl.BaseRequest{
		"id":           "example_ledger_req",
		"command":      "ledger",
		"ledger_index": "validated",
		"transactions": false,
		"expand":       false,
		"owner_funds":  false,
	})
	if err != nil {
		panic(err)
	}

	var ok bool
	index, ok = (resp["result"].(map[string]interface{})["ledger_index"]).(float64)
	if !ok {
		fmt.Printf("ledger_index reflect failed: %T", index)
	}
	

---

another experiment is close twice, and it would be lock at second close, so we need a way to know the connection is close or not or just skip.

	config := xrpl.ClientConfig{
		URL: "wss://s.altnet.rippletest.net:51233/",
	}
	client := xrpl.NewClient(config)
	err := client.Ping([]byte("PING"))
	if err != nil {
		panic(err)
	}

        // 1st
       r.Close()
       
       // 2nd
       r.Close() // will be lock in this close