FIX: Decimal point truncation (#223)

Amounts like $45.80 were being displayed as $45.8. Which doesn't look
correct for currency.

Bug Report: https://meta.discourse.org/t/316007

test/javascripts/helpers/format-currency-test.js

@@ -0,0 +1,17 @@
+import { setupTest } from "ember-qunit";
+import { module, test } from "qunit";
+import { formatCurrency } from "discourse/plugins/discourse-subscriptions/discourse/helpers/format-currency";
+
+module("Unit | Helper | format-currency", function (hooks) {
+  setupTest(hooks);
+
+  test("it formats USD correctly", function (assert) {
+    let result = formatCurrency(["USD", 338.2]);
+    assert.equal(result, "$338.20", "Formats USD correctly");
+  });
+
+  test("it rounds correctly", function (assert) {
+    let result = formatCurrency(["USD", 338.289]);
+    assert.equal(result, "$338.29", "Rounds correctly");
+  });
+});