Add another test case to TestRSA and make it pass

Looks like jsbn doesn't work well with typed arrays. The fix here is straightforward but probably not very good performance-wise, to optimize it we would probably need to modify jsbn (it is not a well documented nor well readable library, so I'd do that only if it will be really necessary).
It seems dangerous to assume the first byte should be zero-padded if the string contains an odd number of characters. Your bytesToHexString implementation won't produce such a string. Have you observed jsbn doing so?