dns: Proxied to become a boolean pointer

[jacobbednarz]

Fixes an inability to send a false proxied value to the API.

type DNSRecord struct {
	Type    string `json:"type,omitempty"`
	Proxied bool   `json:"proxied,omitempty"`
}

func main() {
	a, _ := json.Marshal(DNSRecord{
		Type:    "MX",
		Proxied: true,
	})
	fmt.Println(string(a))
	// => {"type":"MX","proxied":true}
	
	b, _ := json.Marshal(DNSRecord{
		Type:    "MX",
		Proxied: false,
	})
	fmt.Println(string(b))
	// => {"type":"MX"}
}

After swapping to *bool

type DNSRecord struct {
	Type    string `json:"type,omitempty"`
	Proxied *bool  `json:"proxied,omitempty"`
}

func main() {
	a, _ := json.Marshal(DNSRecord{
		Type:    "MX",
		Proxied: &[]bool{true}[0],
	})
	fmt.Println(string(a))
	// => {"type":"MX","proxied":true}

	b, _ := json.Marshal(DNSRecord{
		Type:    "MX",
		Proxied: &[]bool{false}[0],
	})
	fmt.Println(string(b))
	// => {"type":"MX","proxied":false}
}

Fixes #568