Comment 2 for bug 1363722

More detail on where it's hitting in encode_multipart_message() in apiclient/multipart.py

> # Flatten the message without headers.
> buf = BytesIO()
> generator = Generator(buf, False) # Don't mangle "^From".
> generator._write_headers = lambda self: None # Ignore.
> generator.flatten(message)

^^ Failure is happening in the generator.flatten() call.