Allow using the response_headers or headers when getting Rate Limit Info

[DougEdey]

Resolves #1400

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Behavior

Before the change?

• Octokit::TooManyRequests#context.rate_limit methods return nil

After the change?

• Octokit::TooManyRequests#context.rate_limit methods return the correct values

Other information

• I've left this as prioritizing the headers field over response_headers, it's not clear to me when headers was valid (I'm not sure if it was an older version of faraday or something), I looked through the code and I think it's because the on_complete uses response_env (might be here instead of response (Faraday::Response has headers delegating to env.response_headers

Note: I can't see where to add in bug fix information in this repo?

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Additional info

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Added the appropriate label for the given change

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes (Please add the Type: Breaking change label)
• No

If Yes, what's the impact:

• N/A

Pull request type

• Bugfix: Type: Bug

[nickfloyd]

Hey, @DougEdey Thank you for the contribution here! ❤️ It's g2g!