Fix content-type in client_credentials oauth flow

[PushpenderSaini0]

Description

I was getting error while using the client credential oauth flow on login.microsoftonline.com

As per RFC 6749 section-4.4 the client credential grant must have "application/x-www-form-urlencoded" format ! This PR adds the format as a header !

I have verified this fixes the issue 😄 !

• Before
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• After the fix
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Fixes #1987 Fixes #1976 Fixes #1938

Contribution Checklist:

• [X] The pull request only addresses one issue or adds one feature.
• [X] The pull request does not introduce any breaking changes
• [X] I have added screenshots or gifs to help explain the change if applicable.
• [X] I have read the contribution guidelines.
• [X] Create an issue and link to the pull request.