Add option to create HTTP check with IPV6

[duffn]

This adds the ability to create an HTTP check with the IPV6 option checked. This also fixes ip6 -> ipv6 in the CheckResponse struct.

• Docs: https://docs.pingdom.com/api/#tag/Checks/paths/~1checks/post
• Example:

package main

import (
	"fmt"

	"github.com/russellcardullo/go-pingdom/pingdom"
)

func main() {
	client, _ := pingdom.NewClientWithConfig(pingdom.ClientConfig{
		APIToken: "token",
	})

	newCheck := pingdom.HttpCheck{Name: "Test Check", Hostname: "www.google.com", Resolution: 5}
	check, _ := client.Checks.Create(&newCheck)
	fmt.Printf("%+v\n", check)

	ipv6 := true
	newCheck2 := pingdom.HttpCheck{Name: "Test Check 2", Hostname: "www.google.com", Resolution: 5, IPV6: &ipv6}
	check2, _ := client.Checks.Create(&newCheck2)
	fmt.Printf("%+v\n", check2)
}

Closes #69