Have into account the Content-Transfer-Encoding header

Hi @Meatballs1,

https://github.com/Meatballs1/metasploit-framework/pull/35 wasn't smart enough indeed, since we just need to avoid CRLF-ENCODING if the content is binary, as @jlee-r7 pointed again haha soooo hopefully this time it's a better patch for Rex::MIME::Message, @jlee-r7, do you mind to ack?