Correctly fold unknown-8bit originating from encoded words.

The unknown-8bit trick was designed to deal with unknown bytes in an
ASCII message, and it works fine for that.  However, I also tried to
extend it to handle bytes that can't be decoded using the charset
specified in an encoded word, and there it fails because there can be
other non-ASCII characters that were *successfully* decoded.  The fix is
simple: do the unknown-8bit encoding using the utf-8 codec.  This is
especially appropriate since anyone trying to do recovery on an unknown
byte string will probably attempt utf-8 first.

Author: bitdancer

Lib/email/_encoded_words.py

@@ -219,7 +219,7 @@ def encode(string, charset='utf-8', encoding=None, lang=''):
 
     """
     if charset == 'unknown-8bit':
-        bstring = string.encode('ascii', 'surrogateescape')
+        bstring = string.encode('utf-8', 'surrogateescape')
     else:
         bstring = string.encode(charset)
     if encoding is None:

Lib/test/test_email/test__header_value_parser.py

@@ -3255,5 +3255,12 @@ def test_long_filename_attachment(self):
             " filename*1*=_TEST_TES.txt\n",
             )
 
+    def test_encoded_word_with_undecodable_bytes(self):
+        self._test(parser.get_address_list(
+            ' =?utf-8?Q?=E5=AE=A2=E6=88=B6=E6=AD=A3=E8=A6=8F=E4=BA=A4=E7?='
+                )[0],
+            ' =?unknown-8bit?b?5a6i5oi25q2j6KaP5Lqk5w==?=\n',
+            )
+
 if __name__ == '__main__':
     unittest.main()