[graphiql] HTTP customized headers are not sent

[orelus]

API Platform version(s) affected: 3.1.5

Description
When you specify headers in GraphiQL, they are not sent in http headers request.

How to reproduce

Open GraphiQL UI, set a header, open a console with network view, send the request.

No Atuhorization header is present.

Possible Solution

Referer to issue graphql/graphiql#3005 (comment), we need to add headers in graphQLFetcher function in file src/Symfony/Bundle/Resources/public/init-graphiql.js.

function graphQLFetcher(graphQLParams, {headers}) {
    return fetch(entrypoint, {
        method: 'post',
        headers: {
            'Accept': 'application/json',
            'Content-Type': 'application/json',
            ...headers
        },

Now the headers are send.