`'Content-Type': 'application/json'` **needs to be** `'Content-Type': 'application/hal+json'` in order for it to work.

I recommend you watch these video tutorial 

[Learn Drupal 8 - REST API Part1 ](https://www.youtube.com/watch?v=DnZo5lMICKc) 

[Learn Drupal 8 - REST API Part2](https://www.youtube.com/watch?v=Wxoty5nX7BQ)

The login is covered in part 2, but part 1 is very informative and part 2 will make more sense if you watch it.