AbstractRequest amounts MUST include decimals if the currency supports decimals.

Issue tracked here: https://github.com/thephpleague/omnipay-common/issues/17

This rule is to catch issues with legacy code from OmniPay v0.9 OmniPay internally will always return getAmount() as a string formatted to the number of decimal places the currency requires. A gateway that requires a different format (e.g. cents with no decimal rather than dollars) would do the conversion, or overwrite getAmount()