Yeah, that looks like a bug in the old API.  If you try the new API, it does the right thing.  To do that, import email.policy and make your message_as_string call:

  email.message_from_string(raw, policy=email.policy.default)

Note, however, that you really ought to be using message_from_bytes.  Serialized email messages are bytes, not unicode, and using message_from_string will get you in to other trouble.

I don't know if it is worth fixing the old API.