func (p EvenPort) AddTo(m *stun.Message) error {
	v := make([]byte, evenPortSize)
	if p.ReservePort {
		v[0] = firstBitSet
	}
	m.Add(stun.AttrEvenPort, v)
	return nil
}
AddTo adds EVEN-PORT to message.
package proto
import "github.com/pion/stun"
type EvenPort struct {
	ReservePort bool
}
func (p EvenPort) String() string {
	if p.ReservePort {
		return "reserve: true"
	}
	return "reserve: false"
}
const (
	evenPortSize = 1
	firstBitSet  = (1 << 8) - 1  
)