设置头字段中set-cookie为对应cookie  字段类型报错
node版本: v8.1.4
模板引擎: Nunjucks

错误信息:
/middleware/proxy/index.js:390
    ctx.res._headers['set-cookie'] = ctx.res._headers['set-cookie'].concat(cookies);
                                                                                                            ^
TypeError: Cannot read property 'concat' of undefined

