#19063: fix set_payload handling of non-ASCII string input.

This version of the fix raises an error instead of accepting the invalid
input (ie: if a non-ASCII string is used but no charset is specified).

Lib/test/test_email/test_contentmanager.py

@@ -208,12 +208,11 @@ class TestRawDataManager(TestEmailBase):
                          "Basìc tëxt.\n")
 
     def test_get_text_plain_utf8_base64_recoverable_bad_CTE_data(self):
-        m = self._str_msg(textwrap.dedent("""\
+        m = self._bytes_msg(textwrap.dedent("""\
             Content-Type: text/plain; charset="utf8"
             Content-Transfer-Encoding: base64
 
-            QmFzw6xjIHTDq3h0Lgo\xFF=
-            """))
+            QmFzw6xjIHTDq3h0Lgo""").encode('ascii') + b'\xFF=\n')
         self.assertEqual(raw_data_manager.get_content(m, errors='ignore'),
                          "Basìc tëxt.\n")