format the rate in the plan model

test/javascripts/models/plan-test.js.es6

@@ -0,0 +1,17 @@
+import Plan from "discourse/plugins/discourse-patrons/discourse/models/plan";
+
+QUnit.module("discourse-patrons:model:plan");
+
+QUnit.test("subscriptionRate", assert => {
+  const plan = Plan.create({
+    amount: 2399,
+    currency: 'aud',
+    interval: 'month'
+  });
+
+  assert.equal(
+    plan.get("subscriptionRate"),
+    "$23.99 AUD / month",
+    "it should return the formatted subscription rate"
+  );
+});