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).
We should note somewhere that we're using hexStringToBytes because toByteArray doesn't do what we expect.