Improved formatting for outbound messages

[alexdean]

build more standard outbound messages

1. content type application/pkcs7-signature rather than application/x-pkcs7-signature
2. remove plain text "This is an S/MIME signed message" text before first MIME boundary
3. replace plain \n line endings with \r\n

this will be configured via a new property outbound_format on the As2::Config::Partner class.

• v0: our previous format, using OpenSSL::PKCS7.write_smime
• v1: new format incorporating the changes described above

all valid formats will be available via As2::Client.valid_outbound_formats.