[3.7] bpo-36522: Print all values for headers with multiple values. (GH-12681)

[miss-islington]

(cherry picked from commit 461c416)

Co-authored-by: Matt Houglum houglum@google.com

https://bugs.python.org/issue36522

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[miss-islington]

@houglum and @serhiy-storchaka: Status check is done, and it's a success ✅ .

[miss-islington]

@houglum and @serhiy-storchaka: Status check is done, and it's a success ✅ .