这个req.on(‘data’)本身就是一边传一边拼接吧？那如果要使用pipe的话，要在哪个地方createReadStream呢？
能不能用stream和pipe来改写一下以前getPostData的逻辑？？？

//以前的代码
const getPostData = req => {
  const promise = new Promise((resolve, rej) => {
        if (req.method !== 'POST') {
            resolve({})
            return
        }
        if (req.headers['content-type'] !== 'application/json') {
            resolve({})
            return
        }

        let postData = ''
        let chunks = []
        let size = 0
        req.on('data', d => {
            chunks.push(d)
            size += d.length
        })
        req.on('end', () => {
            let buf = Buffer.concat(chunks, size)
            postData = buf.toString()
            if (!!postData) {
                resolve(JSON.parse(postData))
                return
            }
            resolve({})

        })
    })
    return promise
}