fix: fix graphiql request failure

[kiendang]

Fix #1402

After upgrading GraphiQL to v2.4.1 (#1396) all requests seem to fail on Chrome and Safari but works on Firefox. Unfortunately while working on that PR I tested exclusively on Firefox and didn't catch this.

The problem seems to be caused by this line.

graphene-django/graphene_django/static/graphene_django/graphiql.js

Line 78 in 34cc860

headers['Content-Type'] = headers['Content-Type'] || 'application/json';

Also took this opportunity to simplify the GraphQL fetcher logic. Accept and Content-Type are automatically taken care of and don't need setting explicitly.

A more detailed explanation of the bug: content-type: application/json is automatically added and so we have both content-type and Content-Type in the headers object which somehow got turned into content-type: application/json, application/json which is invalid.

@sparktx-adam-gleason @dicknetherlands could you help test this with

pip install git+https://github.com/graphql-python/graphene-django.git@graphiql-headers-fix

[dicknetherlands]

This fix works for me (Chrome on Mac).

[sparktx-adam-gleason]

This fix also works for me (Brave on Mac)

[jaw9c]

Looks good, nice cleanup too 👍