Gmail API fails to send mail using node.js

Question

I followed this tutorial to set up a node.js app that can send emails, but I'm getting the following error when I'm running the code:

Error: 'raw' RFC822 payload message string or uploading message via /upload/* URL required code: 400, errors: [ { domain: 'global', reason: 'invalidArgument', message: '\'raw\' RFC822 payload message string or uploading message via /upload/* URL required' } ]

This is my code:

var fs = require('fs');
var googleAuth = require('google-auth-library');
var {google} = require('googleapis');

function getOAuth2Client(cb) {
  // Load client secrets
  fs.readFile('client_secret.json', function(err, data) {
    if (err) {
      return cb(err);
    }
    var credentials = JSON.parse(data);
    var clientSecret = credentials.installed.client_secret;
    var clientId = credentials.installed.client_id;
    var redirectUrl = credentials.installed.redirect_uris[0];
    var auth = new googleAuth();
    var oauth2Client = new auth.OAuth2(clientId, clientSecret, redirectUrl);

    // Load credentials
    fs.readFile('gmail-credentials.json', function(err, token) {
      if (err) {
        return cb(err);
      } else {
        oauth2Client.credentials = JSON.parse(token);
        return cb(null, oauth2Client);
      }
    });
  });
}

function sendSampleMail(auth, cb) {
  var gmailClass = google.gmail('v1');

  var email_lines = [];

  email_lines.push('From: "test" <my_email@gmail.com>');
  email_lines.push('To: my_email@gmail.com');
  email_lines.push('Content-type: text/html;charset=iso-8859-1');
  email_lines.push('MIME-Version: 1.0');
  email_lines.push('Subject: this would be the subject');
  email_lines.push('');
  email_lines.push('And this would be the content.<br/>');
  email_lines.push('The body is in HTML so <b>we could even use bold</b>');

  var email = email_lines.join('\r\n').trim();

  var base64EncodedEmail = new Buffer(email).toString('base64');
  base64EncodedEmail = base64EncodedEmail.replace(/\+/g, '-').replace(/\//g, '_');

  gmailClass.users.messages.send({
    auth: auth,
    userId: 'me',
    resource: {
      raw: base64EncodedEmail
    }
  }, cb);
}

getOAuth2Client(function(err, oauth2Client) {
  if (err) {
    console.log('err:', err);
  } else {
    sendSampleMail(oauth2Client, function(err, results) {
      if (err) {
        console.log('err:', err);
      } else {
        console.log(results);
      }
    });
  }
});

Is there something I'm doing wrong?