This PR replaces #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 #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
bf838227c3