coap.ServeMux does not support root path ("/") handler

[palsivertsen]

Adding a handler with path "/" will result in a "http: invalid pattern " panic:

package main

import (
	"log"
	"net"

	"github.com/dustin/go-coap"
)

func main() {
	mux := coap.NewServeMux()
	mux.Handle("/", coap.FuncHandler(handler))
	log.Fatal(coap.ListenAndServe("udp", ":5683", mux))
}

func handler(l *net.UDPConn, a *net.UDPAddr, m *coap.Message) *coap.Message {
	log.Printf("%s-%d: %b", a.String(), m.MessageID, m.Payload)
	return nil
}