net/http/httputil: remove all fields in Connection header

[jonathonlacher]

In the reverseproxy, replace use (Header).Get, which returns only one value
of a multiple value header, with using the Header map directly. Also fixes
corresponding tests which hid the bug, and adds more tests.

Fixes #30303

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Message from Emmanuel Odeke:

Patch Set 1:

(3 comments)

Thank you for working on this change Jonathon!

I've added some suggestions but it is important that we keep a mix of the old tests e.g. comma separated header values and also add new ones using header.Add

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Patch Set 1:

(3 comments)

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Patch Set 3:

(1 comment)

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Patch Set 4: Run-TryBot+1

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This PR is being closed because golang.org/cl/166298 has been merged.