Improve fetchBaseQuery default headers handling

[markerikson]

This PR:

• Fixes content-type header handling for non-JSON values like FormData so that the browser can set the header itself
• Sets the accept header if given a specific responseHandler

Fixes #4669
Fixes #3870

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size-limit report 📦

Path	Size
1. entry point: @reduxjs/toolkit/query (cjs, production.min.cjs)	24.41 KB (+0.33% 🔺)
1. entry point: @reduxjs/toolkit/query/react (cjs, production.min.cjs)	26.66 KB (-0.02% 🔽)
2. entry point: @reduxjs/toolkit/query (without dependencies) (cjs, production.min.cjs)	11 KB (+0.12% 🔺)
3. fetchBaseQuery (.modern.mjs)	4.7 KB (+1.46% 🔺)

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