Add order update endpoint (#41)

It allows to update notes, customer, address and so on

order.go

@@ -18,6 +18,7 @@ type OrderService interface {
 	Count(interface{}) (int, error)
 	Get(int, interface{}) (*Order, error)
 	Create(Order) (*Order, error)
+	Update(Order) (*Order, error)
 
 	// MetafieldsService used for Order resource to communicate with Metafields resource
 	MetafieldsService
@@ -314,6 +315,15 @@ func (s *OrderServiceOp) Create(order Order) (*Order, error) {
 	return resource.Order, err
 }
 
+// Update order
+func (s *OrderServiceOp) Update(order Order) (*Order, error) {
+	path := fmt.Sprintf("%s/%d.json", ordersBasePath, order.ID)
+	wrappedData := OrderResource{Order: &order}
+	resource := new(OrderResource)
+	err := s.client.Put(path, wrappedData, resource)
+	return resource.Order, err
+}
+
 // List metafields for an order
 func (s *OrderServiceOp) ListMetafields(orderID int, options interface{}) ([]Metafield, error) {
 	metafieldService := &MetafieldServiceOp{client: s.client, resource: ordersResourceName, resourceID: orderID}

order_test.go

@@ -223,6 +223,30 @@ func TestOrderCreate(t *testing.T) {
 	}
 }
 
+func TestOrderUpdate(t *testing.T) {
+	setup()
+	defer teardown()
+
+	httpmock.RegisterResponder("PUT", "https://fooshop.myshopify.com/admin/orders/1.json",
+		httpmock.NewStringResponder(201, `{"order":{"id": 1}}`))
+
+	order := Order{
+		ID:                1,
+		FinancialStatus:   "paid",
+		FulfillmentStatus: "fulfilled",
+	}
+
+	o, err := client.Order.Update(order)
+	if err != nil {
+		t.Errorf("Order.Update returned error: %v", err)
+	}
+
+	expected := Order{ID: 1}
+	if o.ID != expected.ID {
+		t.Errorf("Order.Update returned id %d, expected %d", o.ID, expected.ID)
+	}
+}
+
 func TestOrderListMetafields(t *testing.T) {
 	setup()
 	defer teardown()