Always return array from clojure-mail.message/message-body

I mean here https://github.com/owainlewis/clojure-mail/blob/b73d63f/src/clojure_mail/message.clj#L158 return `[(msg->map part)]` in order to always be consistent. Since PersistentArrayMap is also sequable currently it may lead to error somewhere deep in stack.

Downside: this is obviously a breaking change :(