Fixed update method for Products repository

no-issue

When updating a Product we can pass existing Stripe Prices, these will
either be adding to the database, or updated if they already exist. When
updating them we were attemping to use the `id` passed in the update,
which is not necessarily included. Instead we should use the `id` of the
StripePrice which we have already retrieved from the database.

ghost/members-api/lib/repositories/product/index.js

@@ -216,7 +216,7 @@ class ProductRepository {
                         nickname: existingPrice.nickname
                     }, {
                         ...options,
-                        id: existingPrice.id || options.id
+                        id: stripePrice.id
                     });
                 }
             }