attributes are not correctly marshaled

[3052]

package main

import (
   "encoding/xml"
   "os"
   nbio "github.com/nbio/xml"
)

func main() {
   var envelope struct {
      XMLName xml.Name `xml:"soap:Envelope"`
      Soap    string   `xml:"xmlns:soap,attr"`
   }
   // <soap:Envelope soap=""></soap:Envelope>
   nbio.NewEncoder(os.Stdout).Encode(envelope)
   // <soap:Envelope xmlns:soap=""></soap:Envelope>
   xml.NewEncoder(os.Stdout).Encode(envelope)
}