Add Content-Type to CldUploadWidget Signature Headers

[colbyfayock]

Description

The CldUploadWidget makes a request to an API endpoint when signing requests and currently doesn't pass along any headers such as Content-Type.

This explicitly defines them as the payload as application/json.

#379

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Fix or improve the documentation
• This change requires a documentation update

Checklist

• I have followed the contributing guidelines of this project as mentioned in CONTRIBUTING.md
• I have created an issue ticket for this PR
• I have checked to ensure there aren't other open Pull Requests for the same update/change?
• I have performed a self-review of my own code
• I have run tests locally to ensure they all pass
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes needed to the documentation

[vercel]

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Name	Status	Preview	Updated (UTC)
next-cloudinary	✅ Ready (Inspect)	Visit Preview	Jan 19, 2024 2:11pm

[github-actions]

🎉 This PR is included in version 6.0.0-beta.4 🎉

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

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