Fixes bug where :body of non multipart messages/emails' bodies is not a vector of maps

I discovered that non multipart emails bodies were different than multipart ones.

A multipart email's `:body`:
```[{"TEXT/PLAIN; charset=us-ascii" "...content..."}
{"TEXT/HTML; charset=ISO-8859-1" "\<html\>...content...\</html/>"}]```
Which is a vector of maps.

A non multipart email's `:body`:
```[["TEXT/PLAIN; charset=us-ascii" "other message"]]```
Which is a vector of vector. Makes it hard to get the message with the text/plain key.

As you can see in the code I simply wrapped (hash-map (content-type... ) with (list ...), to return a list as (map ...) does.