Don't store request specific data into the Layer object

[Congelli501]

Currently, the path & params of the latest request is stored into the last matched layer.
The "Layer" object should be immutable: a request should not modify its attributes.

This means that parameters are kept into memory after the end a request.

For example, this code will show the latest value of the parameter, as the value is stored until a new request is made:

const express = require('./index.js');
const app = express();
const port = 3000;

app.get('/secret/:token', (req, res) => {
	res.send('Hello World!');
});

app.listen(port, () => {
	console.log(`Example app listening on port ${port}`);
});

setInterval(() => {
	console.log(app._router.stack.at(2).params);
}, 1000);

curl localhost:3000/secret/super_secret

Result:

Example app listening on port 3000
undefined
undefined
undefined
{ token: 'super_secret' }
{ token: 'super_secret' }

[dougwilson]

Hello, and thank you for your pull request. Apologies, as I would have mentioned it before if there was an issue, but pull requests against the router files should be made over at https://github.com/pillarjs/router which is synced here (and removed outright in 5.x). If you do reopen, please be sure to include tests cases, as we need those to accept a PR.