Fix copying stack setting in logger's Output method

[adw1n]

Minimum example to reproduce the problem:

package main

import (
	"os"

	"github.com/pkg/errors"
	"github.com/rs/zerolog"
	"github.com/rs/zerolog/log"
	"github.com/rs/zerolog/pkgerrors"
)

func main() {
	log.Logger = log.With().Caller().Stack().Logger().Output(zerolog.ConsoleWriter{Out: os.Stderr})
	zerolog.ErrorStackMarshaler = pkgerrors.MarshalStack

	log.Info().Err(errors.New("test")).Send()
}

Without my changes stack isn't printed.