Fix for issue 4998 - Web Hook Rules check http headers in case sensitive manner

[Rand01ph]

I have add a payload['headers_lower'] for webhook and add a unittest for this.
Fixes #4998

[stale]

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[cognifloyd]

FYI: we just merged a change that reformats all the code with black. Please rebase on master (or merge master into your branch) and reformat your code with black as well. You can use pre-commit install to have black run on your commits. Thanks!

[CLAassistant]

All committers have signed the CLA.

[cognifloyd]

@Rand01ph could you please sign the CLA? (If you signed it before, you'll need to resign it now that it says "Linux Foundation" instead of one of the previous StackStorm stewards).

Also, I just resolved the conflicts, merged master into your branch, and reformatted with black.

I have one style suggestion.

I would like to merge your PR fairly quickly if you can help me at least with the CLA. Hopefully there's not much left before we can merge this.

[arm4b]

Merged! Thanks, everyone! 👍