x509attr: avoid using OpenSSL::ASN1 internals in #value=

[rhenium]

OpenSSL::ASN1 is being rewritten in Ruby. To make it easier, let's remove dependency to the instance variables and the internal-use function ossl_asn1_get_asn1type() outside OpenSSL::ASN1.

This also fixes the insufficient validation of the passed value with its tagging.