Set the charset of the payload to a given character set.
charset can be a Charset instance, a string naming a character set, or
None. If it is a string it will be converted to a Charset instance.
If charset is None, the charset parameter will be removed from the
Content-Type field. Anything else will generate a TypeError.
The message will be assumed to be of type text/* encoded with
charset.input_charset. It will be converted to charset.output_charset
and encoded properly, if needed, when generating the plain text
representation of the message. MIME headers (MIME-Version,
Content-Type, Content-Transfer-Encoding) will be added as needed.