fix endless while cycles with locales not using a thousands separator, add Belarus to countries list

Two fixes here:
- When locale settings are such that the thousands separator is an empty string (no separator is used as per locale convention — is the case with Latvian, Croatian, Polish, Dutch and many other locales), Recurly.js would cause the entire page to stall due to a neverending while cycle. The fix would be to return the original sum string immediately instead.
- Belarus was missing from countries list, added it there.