Allow passing custom headers to metadata request in authorize_redirect

[AlexandrosMaragkakis]

This pull request implements a feature to allow custom HTTP headers to be passed to the metadata request made by authorize_redirect().

Previously (#633), when calling authorize_redirect(redirect_uri, headers=headers), the custom headers were not forwarded to the GET request for server metadata, causing a 401 Unauthorized error when the endpoint was protected by a security gateway.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

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[azmeuk]

Hello. Thank you for your contribution. Would you consider adding some unit tests?

[AlexandrosMaragkakis]

Hello. Unfortunately I don't have any experience in unit testing. Having said that, I am willing to give it a try. Perhaps you could suggest a file from the tests of the project, to serve as an example for me?

[azmeuk]

You could take inspiration from this test, register a client with custom headers, and then check if the mocked send method is called with your custom headers.
If you are not familiar with mock, I guess the subject is pretty well covered on the internet.

[azmeuk]

Is this needed as client_kwargs is automatically initialized as an empty dict when a None value is passed?

authlib/authlib/integrations/base_client/sync_app.py

Line 217 in da87c8b

self.client_kwargs = client_kwargs or {}

[AlexandrosMaragkakis]

You are correct, thanks for pointing it out. I also cleaned the other changes a bit.

[AlexandrosMaragkakis]

I appreciate the guidance. I'll get on with adding the tests.