Return a reference to the payload.
The payload will either be a list object or a string. If you mutate
the list object, you modify the message's payload in place. Optional
i returns that index into the payload.
Optional decode is a flag indicating whether the payload should be
decoded or not, according to the Content-Transfer-Encoding header
(default is False).
When True and the message is not a multipart, the payload will be
decoded if this header's value is `quoted-printable' or `base64'. If
some other encoding is used, or the header is missing, or if the
payload has bogus data (i.e. bogus base64 or uuencoded data), the
payload is returned as-is.
If the message is a multipart and the decode flag is True, then None
is returned.