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