gh-80361: Fix TypeError in email.Message.get_payload() (GH-117994)

It was raised when the charset is rfc2231 encoded, e.g.:

   Content-Type: text/plain; charset*=ansi-x3.4-1968''utf-8

Lib/email/message.py

@@ -294,7 +294,7 @@ class Message:
                 try:
                     bpayload = payload.encode('ascii', 'surrogateescape')
                     try:
-                        payload = bpayload.decode(self.get_param('charset', 'ascii'), 'replace')
+                        payload = bpayload.decode(self.get_content_charset('ascii'), 'replace')
                     except LookupError:
                         payload = bpayload.decode('ascii', 'replace')
                 except UnicodeEncodeError: