fix(useFetch): generated payloadType on execute

[tolking]

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As the VueUse audience continues to grow, we have been inundated with an overwhelming number of feature requests and pull requests. As a result, maintaining the project has become increasingly challenging and has stretched our capacity to its limits. As such, in the near future, we may need to slow down our acceptance of new features and prioritize the stability and quality of existing functions. Please note that new features for VueUse may not be accepted at this time. If you have any new ideas, we suggest that you first incorporate them into your own codebase, iterate on them to suit your needs, and assess their generalizability. If you strongly believe that your ideas are beneficial to the community, you may submit a pull request along with your use cases, and we would be happy to review and discuss them. Thank you for your understanding.

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Description

close #3250

Additional context

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🤖 Generated by Copilot at f85bdf3

Improved the handling of different payload types and headers in useFetch and added tests to verify the functionality. Refactored the code to avoid duplication and ensure consistency.

🤖 Generated by Copilot at f85bdf3

• Move the logic for setting the payload type and the Content-Type header to the main useFetch function, to ensure consistency and avoid duplication (link, link)
• Modify the condition for setting the payload type to json to exclude FormData instances, since they are handled by fetch without specifying the header (link)
• Add two test cases to check the behavior of useFetch with different types of payloads (JSON and FormData) and how the Content-Type header is set accordingly (link)
• Use the ref function from Vue to create reactive values for the payload and the retry function from the test utils to wait for the fetch call to complete and assert the expected header value in the test cases (link)