default_invoice_memo should be an Option

[sjmiller609]

On a Subscription, the default_invoice_memo should be an Option. It will use the Subscription's memo if that exists, otherwise use the Plan's memo if that exists, otherwise the server returns null.

rust-orb-billing/src/client/subscriptions.rs

Line 140 in 87d6697

pub default_invoice_memo: String,

This problem does not occur when Subscriptions include memos or when Plans include memos. When neither includes a memo, then the client will fail to deserialize the response, for example in the case of create_subscription it will fail to deserialize and error out, but the request works.

rust-orb-billing/src/client/subscriptions.rs

Lines 267 to 279 in 87d6697

	pub async fn create_subscription(
	&self,
	subscription: &CreateSubscriptionRequest<'_>,
	) -> Result<Subscription, Error> {
	let mut req = self.build_request(Method::POST, SUBSCRIPTIONS_PATH);
	if let Some(key) = subscription.idempotency_key {
	req = req.header("Idempotency-Key", key);
	}
	let req = req.json(subscription);
	let res = self.send_request(req).await?;
	Ok(res)
	}

I noticed this while adding my test for cancel subscription because in the test file, it assumes the existence of a plan in Orb.

rust-orb-billing/tests/api.rs

Line 591 in 87d6697

plan_id: orb_billing::PlanId::External("test"),

I did not have that plan, so I created it. That reproduced this issue. Since this issue is not related to adding cancel subscription to this SDK, I just added a memo on my Plan to avoid the issue in that PR.