'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
Learn Drupal 8 - REST API Part2
The login is covered in part 2, but part 1 is very informative and part 2 will make more sense if you watch it.