gh-123742: Fixed get_payload not being able to parse headers with spaces

[rruuaanng]

• Issue: Make email/message.py read headers more robustly #123742

After getting the email encoding format, added strip() to remove unexpected spaces

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[erlend-aasland]

@rruuaanng, I will point again to the devguide and suggest that you read it again. The Getting Started section states that we avoid force-pushing; it messes up the review process and does not play well with CI feedback.

[erlend-aasland]

Previous review remarks were not addressed; please address them.

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[erlend-aasland]

Can you please add a test for this?

[rruuaanng]

It has been a long time since this PR was opened. If there are no problems, I hope we can approve it :)

[rruuaanng]

I discussed this with Bénédikt in #123742 and got some feedback. I think we can follow the conclusion of practice, which is to remove trailing spaces, otherwise it will cause parsing errors, even though the RFC is vague about this, but I think making the module work properly is our ultimate pursuit.

[rruuaanng]

Thank you, Erlend, for your repeated review of this. But now, I will close this PR, It has been solved.