test_email: content-shadow header tuple is discarded in test_message.py #161479

Bug report

The tuple is created and immediately discarded, so content-shadow never ends
up in msg_headers and the check that non-standard content-* headers (other
than Content-Type) are copied to the part in _check_make_multipart never
runs:

The mistake masks a real regression: if the condition in Lib/email/message.py
is narrowed to Content-Type only, the current test set stays green (307 OK),
while with the content-shadow header properly appended 36 tests fail.

Found by Linux Verification Center (linuxtesting.org) with SVACE

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-161479: Fix discarded header tuple in test_email test_message #161480