func (r *multipartReader) NextPart() (*Entity, error) {
	p, err := r.r.NextPart()
	if err != nil {
		return nil, err
	}
	return New(Header{p.Header}, p)
}
NextPart implements MultipartReader.
package message
import (
	"io"
	"github.com/emersion/go-message/textproto"
)
type MultipartReader interface {
	io.Closer
	NextPart() (*Entity, error)
}
type multipartReader struct {
	r *textproto.MultipartReader
}