fixed add to shoppinglist endpoint

src/main/java/ntnu/idatt2016/v233/SmartMat/service/ShoppingListService.java

@@ -78,6 +78,7 @@ public class ShoppingListService {
         shoppingListRepository.findById(shoppingListId).ifPresent(shoppingList -> {
             productRepository.findById(ean).ifPresent(product -> {
                 shoppingList.addProduct(product);
+                product.addShoppingList(shoppingList);
                 shoppingListRepository.save(shoppingList);
             });
         });