improve MDN construction

[alexdean]

have experienced reports of partners being unable to verify MDNs created by our code. PR incorporates several fixes.

for backards-compatibility, the existing MDN format will remain the default even though it has known defects. the new format will be an opt-in option on a per-partner basis.

new configuration

partner = As2::Config::Partner.new
partner.mdn_format = 'v0' # (or 'v1')

As2::Server.valid_mdn_formats
# => ["v0", "v1"]

formats

v0 format

• built by OpenSSL::PKCS7.write_smime
• uses content type application/x-pkcs7-signature
• contains MIME headers (MIME-Version and Content-Type) in the message body.
• includes a text "This is an S/MIME message" in the message body, before the first MIME boundary
• uses \n for line endings

v1 format

• uses content type application/pkcs7-signature
• contains no text before the first MIME boundary
• uses \r\n for line endings