rfc6979: failure to replicate NIST P-521 test vectors

[tarcieri]

P-521 is of course an unusual edge case in that it has an odd(ly shaped) field size and one that exceeds the size of e.g. SHA-512.

#775 unsuccessfully attempts to add the test vector.

[tarcieri]

Fixed in #781, which added a missing required right shift in the event the modulus's bit length and byte length aren't aligned