The problem only arises with HeaderParser.  The full parser turns the body into a list containing a Message object, because that's how the email package models the message structure.  HeaderParser treats the body as a single string.  All that is fine, but generator's flatten method has special handlers for certain mime types, and message/rfc822 is one of them.  It is looking for what it "knows" the message/rfc822 body looks like (a list containing a Message), but what it finds is a string.

The fix is probably to special case get_payload returning a string inside the _handle_message.  I've attached a patch that adds your test file as a unit test and provides that fix.