Subscription upgrade with 'proration-none' incorrectly charges immediately

When upgrading a subscription with `update_behavior` set to `proration-none`, the API is still charging immediately. This is not the expected behavior. The user provided the following cURL request that demonstrates the issue: `curl --request POST \ --url https://test-api.creem.io/v1/subscriptions/sub_2ZQu4t82t82Iv2A45xv2nY/upgrade \ --header 'Content-Type: application/json' \ --header 'x-api-key: creem_test_5MrUueNVkJVeRKxUnJ20q5' \ --data ' { "product_id": "prod_6t3fsD4JXrJfw1RJfUJnto", "update_behavior": "proration-none" } '`