issue #953: stop using writeHead

Created by: jakefurler

Using object.keys in web-incoming.js results in a non-deterministic ordering of keys, which means that in web-outgoing writeHead might be called before setHeader, which throws an error.

Example of a server that fails to send a response :

const http = require('http');

const server = http.createServer((req, res) => {
  res.writeHead(200);
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.end('ok');
});

server.listen(3000);

This will throw Error: Can't set headers after they are sent. when a request is made to it. If we change it to the following, it will work:

const http = require('http');

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  // res.writeHead(200);
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.end('ok');
});

server.listen(3000);