header(name, value) {
    if (value === undefined && helper.isString(name)) {
      return this.ctx.header[name];
    }
    if (this.ctx.res.headersSent) {
      debug(`headers has already sent, url: ${this.ctx.url}`);
      return;
    }
    if (value !== undefined) {
      return this.ctx.set(name, value);
    }
    if (helper.isObject(name)) {
      return this.ctx.set(name);
    }
  }
get or set header
@param {String} name
@param {Mixed} value