Money.add_rate spec should not impact default_bank

Otherwise other specs that do not expect a EUR -> USD rate to exist may get unexpected behavior

If we don't isolate the changes to the default_bank's rates, another spec with the following assertion may intermittently fail (based on the order specs are run):
```
expect { Money.new(10_00, "EUR").exchange_to("USD") }.to raise_error(Money::Bank::UnknownRate)
```