code: function getBody(context, config, stats, pathname) {
    let _stream
    // 判断是否range请求
    const hasRange = context.get('Range')
    if (hasRange) {
      let range = getRange(hasRange, stats.size)
      if (range !== -1 && range !== -2) {
        // 设置响应头
        context.set({
          'Accept-Ranges': range.AcceptRanges,
          'Content-Range': range[0].ContentRange
        })
        // 设置正文长度
        context.length = range[0].ContentLength
        context.status = 206
        _stream = fs.createReadStream(pathname, {
          start: range[0].start,
          end: range[0].end
        })
      } else {
        // 所请求的范围无法满足 (Requested Range not satisfiable)
        context.status = 416
        return
      }
    } else {
      context.length = stats.size
      _stream = fs.createReadStream(pathname)
    }
    return _stream
  }
docs: 获取响应正文body
  @param  {Object} config   静态服务器中间件配置
  @param  {Object} stats    文件信息
  @param  {String} pathname 文件路径
  @return {Stream}          响应正文