I don't actually like this so much to check if the expire_rates (and same for the update_rates at line 296). I tried to use the assert_send but it requires the method to return true, I stubbed it to return true, but then it always pass even when I remove the calling in the method itself... I tried to use mocha an using the .expects method, it works, but all other tests fail. I haven't check out much about it.

If you have any better option feel free to improve it; it's actually the first time I use minitest, I usually use rspec :)