Fix cart merging - old cart should merge into new one (#1920)

This PR fixes cart merging (`lunar.cart.auth_policy: merge`) to ensure
that the current cart session is the one that has the line items applied
to it, before this PR it was being applied to the old cart which didn't
make sense.

packages/core/src/Actions/Carts/AssociateUser.php

@@ -18,7 +18,7 @@ public function execute(Cart $cart, LunarUser $user, $policy = 'merge'): self
         if ($policy == 'merge') {
             $userCart = Cart::whereUserId($user->getKey())->active()->unMerged()->latest()->first();
             if ($userCart) {
-                app(MergeCart::class)->execute($userCart, $cart);
+                app(MergeCart::class)->execute($cart, $userCart);
             }
         }