Updates an user

{
  "data": {
    "attributes": {
      "email": "user@commercelayer.io",
      "first_name": "John",
      "last_name": "Doe",
      "time_zone": "UTC",
      "reference": "ANY-EXTERNAL-REFEFERNCE",
      "reference_origin": "ANY-EXTERNAL-REFEFERNCE-ORIGIN",
      "metadata": {
        "foo": "bar"
      }
    }
  }
}