Fetch post with body data not working params empty

Followed this topic on github: https://github.com/matthew-andrews/isomorphic-fetch/issues/34

The solution to my question is using the JSON.stringify function and set Content-Type header to application/json. Not pretty sure why the second attempt in my question didn’t work though.

fetch('/api/v1/users', {  
    method: 'post',
    headers: {'Content-Type': 'application/json'},
    body: JSON.stringify({ "user": {
      "email" : email,
      "password" : password
    }}),
})

Official MDN Documentation:

var myHeaders = new Headers();
myHeaders.append('Content-Type', 'application/json');

fetch('/contact-form', {
    method: 'POST',
    headers: myHeaders,
    mode: 'cors',
    cache: 'default',
    body: JSON.stringify(fields)
}).then(() => {
    dispatch(contactFormSubmitSuccess());
});