func (p Problem) XML() []byte {
	b, _ := xml.Marshal(p)
	return b
}
XML returns the Problem as json bytes
package problem
import (
	"encoding/json"
	"encoding/xml"
	"fmt"
	"net/http"
	"strconv"
)
const (
	ContentTypeJSON = "application/problem+json"
	ContentTypeXML = "application/problem+xml"
)
type Option interface {
	apply(*Problem)
}
type optionFunc func(*Problem)
func (f optionFunc) apply(problem *Problem) { f(problem) }
type Problem struct {
	data   map[string]interface{}
	reason error
}
func (p Problem) JSON() []byte {
	b, _ := p.MarshalJSON()
	return b
}