Cannot enable my own SSL certificate on heroku

I host my website at heroku using my own domain name using node.js & express 4.x. I have purchased SSL certificate and I want to add it to my website.

I use the following code to enable HTTPS support:

var fs = require('fs');
var http = require('http');
var https = require('https');
var privateKey  = fs.readFileSync('sec/private_server.key', 'utf8');
var certificate = fs.readFileSync('sec/server.crt', 'utf8');

var credentials = { key: privateKey, cert: certificate };


// launch http server
var httpServer = http.createServer(app).listen( process.env.PORT, process.env.IP || "0.0.0.0", function() {
    console.log('Listening on port %d', process.env.PORT);
});


// launch https server
var httpsServer = https.createServer(credentials, app).listen( 8443, process.env.IP || "0.0.0.0", function() {
    console.log('Listening HTTPS on port 8433' );
});

Launching the server and visiting it by https link shows that I this website still uses heroku certificate.

What do I do wrong?