gh-160302: Don't encode the payload twice in Message.set_payload()

When the input and output charsets differ, as for euc-jp and shift_jis,
which are sent as iso-2022-jp, set_payload() encoded the text and
set_charset() then encoded the result again. That only worked because
the iso2022_jp encoder let ESC through. Leave the conversion to
set_charset().

Author: aozmekik

Lib/email/message.py

@@ -352,7 +352,11 @@ def set_payload(self, payload, charset=None):
                 return
             if not isinstance(charset, Charset):
                 charset = Charset(charset)
-            payload = payload.encode(charset.output_charset, 'surrogateescape')
+            # If the input and output charsets differ, set_charset()
+            # converts the text, so don't encode it twice.
+            if charset == charset.get_output_charset():
+                payload = payload.encode(charset.output_charset,
+                                         'surrogateescape')
         if hasattr(payload, 'decode'):
             self._payload = payload.decode('ascii', 'surrogateescape')
         else: