add an OpenSSL::PKCS5 test
avoiding Krypt gem dependency completely (was used for OpenSSL::PKCS5)

also Krypt delegated `pbkdf2_hmac` was not compatible with MRI's OpenSSL ...

NOTE: krypt was embedded into JRuby since 1.7.3 https://github.com/jruby/jruby/commit/cc9acbaf23ac5d
PKCS5 cleanup and make sure we test for passing in a string digest algorithm name