title: "Unprocessable entity" when trying to change the fulfilment status of an order IN SHOPIFY
body: <p>I m trying to change the fulfilment status of an order , this is the json
  data sent :</p>
  
  <p>{"fulfillment":{"tracking_number":null,"line_items":[{"id":"XXXXXXX"}]}} and this is the url "/admin/orders/XXXXXXXXX/fulfillments.json"
  but i get this error "Unprocessable entity" .</p>
  
  <p>looking for your help .</p>
  
  <p>thanks
  Alaeddine </p>