add specs around the payment callback

spec/models/payment_spec.rb

@@ -1,4 +1,27 @@
 require 'rails_helper'
 
 RSpec.describe Payment do
+  describe '#update_bill_attributes' do
+    context 'when the bill is non-recurring' do
+      let(:payment) do
+        payment = create(:payment)
+        payment.bill.update(repeat_interval: create(:repeat_interval, interval: 'one_time'))
+        payment
+      end
+
+      it 'marks the bill as inactive' do
+        payment.update_bill_attributes
+        expect(payment.bill.inactive?).to be_truthy
+      end
+    end
+
+    context 'when the bill is recurring' do
+      let(:payment) { create(:payment) }
+
+      it 'updates the bills due date' do
+        payment.update_bill_attributes
+        expect(payment.bill.next_due_date.strftime('%m/%d/%Y')).to eql(2.months.since.strftime('%m/%d/%Y'))
+      end
+    end
+  end
 end