Fix fetchJSON adds useless Content-Type header for empty requests

[chrisDeFouRire]

I'm using Fastify on the backend, and it refuses DELETE requests sent from react-admin's ra-data-json-server.
It is because fetchUtils.fetchJson is setting the Content-Type header to application/json, even with an empty body like in the delete requests. The function that is setting the header is createHeadersFromOptions in fetch.ts.

It is not against the RFCs to do so, but it just breaks Fastify, and it makes little sense to send a Content-Type with a body that doesn't adhere to the content type: an empty body isn't valid json.

This pull request fixes the issue and adds new unit tests to ensure there is no regression.

yarn test-unit -- ./packages/ra-core/src/dataProvider/fetch.spec.ts passes.

I guess the alternatives are:

• sending a body if there isn't one already (with 'null' in it)
• modifying the ra-data-json-server package (and other data providers which are using DELETE with no body) to specify a body.

I thought fixing the createHeadersFromOptions option was the cleanest

[fzaninotto]

Nice!

[chrisDeFouRire]

Thank you for giving us React Admin! it's a great tool ! and seeing how easily I can contribute makes me think it's the perfect tool for us even more! 🙏🙏

[fzaninotto]

Thanks for your PR, and for your kind words!