Compare by the dotted decimal notation rather than the NID.

OpenSSL::ASN1::ObjectId can store OIDs that are not registered in
OpenSSL's internal table. NID is not defined for such an OID, but it is
not an error.

The == method also should not raise TypeError if the other object is
not an instance of OpenSSL::ASN1::ObjectId.

Fixes: https://github.com/ruby/openssl/issues/791