[3.15] Add yet one assertion in test_set_text_charset_cp949 (GH-151224) (GH-151237)

Add yet one assertion in test_set_text_charset_cp949 (GH-151224)

Check bytes(m), not only str(m).
(cherry picked from commit 3ca93ab)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: miss-islington

Lib/test/test_email/test_contentmanager.py

@@ -352,7 +352,14 @@ def test_set_text_charset_cp949(self):
 
             x9Gxub7uCoFBCg==
             """))
-        self.assertEqual(m.get_payload(decode=True).decode('ks_c_5601-1987'), content)
+        self.assertEqual(bytes(m), textwrap.dedent("""\
+            Content-Type: text/plain; charset="ks_c_5601-1987"
+            Content-Transfer-Encoding: 8bit
+
+            \ud55c\uad6d\uc5b4
+            \uac02
+            """).encode('ks_c_5601-1987'))
+        self.assertEqual(m.get_payload(decode=True), content.encode('ks_c_5601-1987'))
         self.assertEqual(m.get_content(), content)
 
     def test_set_text_plain_long_line_heuristics(self):