bpo-36522: Print all values for headers with multiple values

[houglum]

This is a follow-up to https://bugs.python.org/issue33365. The fix for that issue (see #6611) added a statement to also print header values, but it does not account for the case where multiple values exist for the same header name (note that multiple header values for the same header name can be supplied in one comma-separated entry, or in multiple entries; see this StackOverflow post). E.g. if my response contained these headers:

x-goog-hash: crc32c=KAwGng==
x-goog-hash: md5=eB5eJF1ptWaXm4bijSPyxw==

then the debug output would print whichever of those values is returned from self.headers.get("x-goog-hash") for both prints:

header: x-goog-hash: crc32c=KAwGng==
header: x-goog-hash: crc32c=KAwGng==

This PR changes the header iteration to use self.headers.items() instead, which will return the key and value pair to be printed, resulting in the correct debug output:

header: x-goog-hash: crc32c=KAwGng==
header: x-goog-hash: md5=eB5eJF1ptWaXm4bijSPyxw==

https://bugs.python.org/issue36522

[the-knights-who-say-ni]

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[houglum]

Just signed the CLA and added a NEWS entry.

[miss-islington]

Thanks @houglum for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[serhiy-storchaka]

Thank you for your contribution @houglum!

[bedevere-bot]

GH-12682 is a backport of this pull request to the 3.7 branch.