Sandbox mode patch

The SandBoxMode field is now located in MessagesV31 struct

If you want to use the sandbox mode to test your emails validity, below is an example

package main

import (
	"fmt"
	"log"
	"os"

	mailjet "github.com/mailjet/mailjet-apiv3-go"
)

func main() {
	m := mailjet.NewMailjetClient(
		os.Getenv("MJ_APIKEY_PUBLIC"),
		os.Getenv("MJ_APIKEY_PRIVATE"))

	messagesInfo := []mailjet.InfoMessagesV31{
		mailjet.InfoMessagesV31{
			From: &mailjet.RecipientV31{
				Email: "qwe@qwe.com",
				Name:  "Bob Patrick",
			},
			To: &mailjet.RecipientsV31{
				mailjet.RecipientV31{
					Email: "qwe@qwe.com",
				},
			},
			Subject:  "Send API v 3.1 testing",
			TextPart: "Hey, that's pretty good!",
		},
	}

	messages := mailjet.MessagesV31{Info: messagesInfo, SandBoxMode: true}

	res, err := m.SendMailV31(&messages)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("Data: %+v\n", res)
}