客户端(桶) 水管 服务端（桶）
    GET请求 <-
    获取数据
    POST请求 ->
    发送数据

    1. const http = require('http');
    2. const querystring = require('querystring');
    4. const server = http.createServer((req, res) => {
    5.   if (req.method === 'POST') {
    6.     let postData = '';
    7.     // 流 stream
    8.     req.on('data', chunk => {
    9.       postData += chunk.toString();
    10.     })
    12.     req.on('end', () => {
    13.       console.log('postData', postData);
    14.       res.end('数据接收完毕');
    15.     })
    17.     console.log('post data content type', req.headers['content-type']);
    18.   }
    19. });
    21. server.listen(5000, () => {
    22.   console.log('server running at port 5000');
    23. })