- Allow charset header to be set in content-type passed to Attachment
  constructor.

- Don't permit a text body to be set when creating a non-text attachment.

- Add an attach_binary method to a response.

- Encode the body properly on Python 2 in the various attach_* methods in a
  response.

- Encode the body properly within the to_message method of a response.

- Transfer-encode the body properly within the extract_payload method of a
  response.

- About to lose my shit.