ASN1_dup() will not copy the 'pkey' field of a PKCS7_SIGNER_INFO object
by design; it is a temporary field kept until the PKCS7 structure is
finalized. Let's bump reference counter of the pkey in the original
object and use it in the new object, too.

This commit also removes PKCS7#add_signer's routine to add the
content-type attribute as a signed attribute automatically. This
behavior was not documented or tested. This change should not break any
working user code since the method was completely useless without the
change above.

https://github.com/ruby/openssl/commit/20ca7a27a8