bpo-27321 Fix email.generator.py to not replace a non-existent header. (GH-18074)

This PR replaces GH-1977. The reason for the replacement is two-fold.

The fix itself is different is that if the CTE header doesn't exist in the original message, it is inserted. This is important because the new CTE could be quoted-printable whereas the original is implicit 8bit.

Also the tests are different. The test_nonascii_as_string_without_cte test in GH-1977 doesn't actually test the issue in that it passes without the fix. The test_nonascii_as_string_without_content_type_and_cte test is improved here, and even though it doesn't fail without the fix, it is included for completeness.

Automerge-Triggered-By: @warsaw
(cherry picked from commit bf838227c3)

Co-authored-by: Mark Sapiro <mark@msapiro.net>

Lib/email/generator.py

@@ -186,7 +186,11 @@ class Generator:
         # If we munged the cte, copy the message again and re-fix the CTE.
         if munge_cte:
             msg = deepcopy(msg)
-            msg.replace_header('content-transfer-encoding', munge_cte[0])
+            # Preserve the header order if the CTE header already exists.
+            if msg.get('content-transfer-encoding') is None:
+                msg['Content-Transfer-Encoding'] = munge_cte[0]
+            else:
+                msg.replace_header('content-transfer-encoding', munge_cte[0])
             msg.replace_header('content-type', munge_cte[1])
         # Write the headers.  First we see if the message object wants to
         # handle that itself.  If not, we'll do it generically.