Send GitHubRequestAcceptHeader.BASE_JSON as a string

[srt32]

Proposed change

Today, I think we are sending our internal enum name instead of the header value we mean as a string. This change casts it to a string when we send it.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionalit)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:

Checklist

• The code change is tested and works locally.
• Local tests pass.
• There is no commented out code in this PR.
• The code has been formatted using Black (make lint)
• Tests have been added to verify that the new code works.

[ludeeus]

Thanks 👍