HiTrust doesn't use ISO4217 definition of TWD

[bdewater]

We've had merchants report that although TWD officially has cents, in practice nobody uses it. According to Wikipedia "Subunits used only in stocks and currencies, and rarely referred to in such cases."

HiTrust apparently only accepts TWD as a fractionless currency, like JPY/ISK. Imperfect as the ISO standard may be, it's the only thing that is mutually agreed upon and users of the gem should be able to trust that we're accepting and returning sane values. A gateway diverging from this needs to handle this idiosyncrasy internally.