message/rfc822 it turns it into an object whose body consists of
a list containing a single Message object.  HeaderParser, on the
other hand, just copies the body as a string.  Generator.flatten
has a special handler for the message mime type that expected the
body to be the one item list.  This fails if the message was parsed
by HeaderParser.  So we now check to see if the body is a string
first, and if so just we just emit it.