OpenSSL::PKCS7::SignerInfo#signed_time unimplemented

Was hoping to get hexapdf working with JRuby but the `OpenSSL::PKCS7::SignerInfo#signed_time` method is currently unimplemented.

Any chance this could be implemented?

I don't know much about the method itself, but the C code calls `PKCS7_get_signed_attribute(p7si, NID_pkcs9_signingTime)` under the hood.