Incorrect headers being set on DELETE request when using fetchUtils from ra-data-simple-rest

[rahulbhanushali]

What you were expecting:

DELETE request should be not fired with Content-Type header set to application/json.

What happened instead:

DELETE request is being fired with Content-Type header set to application/json.
DELETE requests are usually fired with empty body and that is happening correctly with ra-data-simple-rest as well but the only problem is Content-Type request header is being set which shouldn't be case.
This fails with frameworks like fastify. Related discussion here: fastify/fastify#297 (comment).

Steps to reproduce:

Delete an entity from the UI and observe the HTTP request headers.

Other information:

This is happening mainly because of the assumption in the code that all requests should be fired with Content-Type set to application/json in the fetch.ts file.

export const createHeadersFromOptions = (options: Options): Headers => {
    const requestHeaders = (options.headers ||
        new Headers({
            Accept: 'application/json',
        })) as Headers;
    if (
        !requestHeaders.has('Content-Type') &&
        !(options && (!options.method || options.method === 'GET')) &&
        !(options && options.body && options.body instanceof FormData)
    ) {
        requestHeaders.set('Content-Type', 'application/json');
    }
    if (options.user && options.user.authenticated && options.user.token) {
        requestHeaders.set('Authorization', options.user.token);
    }

    return requestHeaders;
};

Environment

• React-admin version: 3.9.4
• Last version that did not exhibit the issue (if applicable):
• React version: 16.14
• Browser: Chrome
• Stack trace (in case of a JS error):

[fzaninotto]

I'd say that it's not really incorrect - the HTTP standard doesn't force any content type for responses with an empty body. I'd say that a strict interpretation of the HTTP standard by some server libraries have them decode the body as soon as the content-type says the content is JSON, but that's their problem.

That being said, it doesn't do any harm to change the content-type header to text/plain for delete in ra-data-simple-rest.

I'm marking this as a bug, feel free to submit a PR to fix it!