urllib3: Fix headers when used in kwargs

[kjprice]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes #4115

Type of change

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• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please see this discussion on how to test.

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

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[kjprice]

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[herin049]

I'd prefer that we use a more robust method to accomplish this. I'd suggest we use a combination of inspect.signature and signature.bind to bind the arguments.

[herin049]

We also need to add tests for this as well. You mentioned you don't have a large amount of time to contribute so I've implement the changes using inspect.signature with signature.bind and additional tests here: #4144

[kjprice]

Yay! Thanks @herin049!

[rite7sh]

hey, it might be worth adding a regression test that explicitly passes headers positionally with instrumentation enabled, just to lock this behavior in and avoid regressions? Otherwise this approach looks solid to me