dont serialize null as header values

[Uzlopak]

fixes #249

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Fdawgs]

This got me as I use undefined in my header tests, thanks for the fix!

[mcollina]

Technically this does not match node core behavior. It serializes null as 'null', and this module mimics core as much as possible.

[Fdawgs]

Technically this does not match node core behavior. It serializes null as 'null', and this module mimics core as much as possible.

Ah balls, what about undefined?

[Fdawgs]

Technically this does not match node core behavior. It serializes null as 'null', and this module mimics core as much as possible.

Ah balls, what about undefined?

@mcollina @Uzlopak thoughts on this?

[mcollina]

const { createServer, request } = require('http');

const server = createServer((req, res) => {
  res.setHeader('Content-Type', 'text/plain');

  try {
    res.setHeader('X-undefined', undefined);
  } catch (err) {
    console.log('undefined could not be set')
  }
  res.setHeader('X-null', null);
  res.end('Hello World!');
})

server.listen(3000, function () {
  console.log('Server listening on port 3000');
  request('http://localhost:3000', function (res) {
    console.log(res.headers);
    process.exit(0);
  }).end()
});

[Fdawgs]

Aight, cool.

I was using undefined in my tests like so to remove expected headers that I... no longer expected:

const sitewideExpectedHeaders = {
	  'Content-Type': 'application/json',
	  expires: "0",
}

const cacheablepageExpectedHeaders = {
	...sitewideExpectedHeaders,
	expires: undefined
}

Guess I'll need to just change it to use delete cacheablepageExpectedHeaders.expires instead.