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 bf838227c35212709dc43b3c3c57f8e1655c1d24)

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

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