let config = {
  method: 'POST',
  headers: {
    "Accept": "application/json",
    "Content-Type": "application/json"
  },
  body: JSON.stringify(creds) // { email: 'asd@example.org', password: 'asdasdasd' }
}

return dispatch => {
    dispatch(requestLogin(creds))
    return fetch('http://localhost:3000/auth/sign_in', config)
      .then(response =>
        response.json()
        .then(user => ({ user, response }))
      ).then(({ user, response }) =>  {
        if (!response.ok) {
          // ...
        }
        else {
          console.log(response.headers.get('access-token')) // => null
        }
      }).catch(err => console.log("Error: ", err))
}