Bugfix #1119552: PKCS#1v1.5 has to accept signatures without NULL parameters

The digest AlgorithmIdentifier has optional (NULL) parameters; the
verification function should not reject a signature if they are omitted.

With this fix, either case is acceptable (parameters present with value NULL
or not present).

As an exception, signatures based on old MD2/MD5 must always have NULL
parameters.

See Appendix B.1 of RFC 3447 and Section 2.1 of RFC 4055.
Added unit tests for bugfix #1119552