fix: send json accept headers for POST requests

[mariajgrimaldi]

Description

This PR solves a CORS error when calling the endpoints for Changing Site Language and Unlinking the SSO Account. To resolve this error we simply specify the Accept: 'application/json' header in the request.

These are the related issues:

#1052
openedx/wg-build-test-release#376

#1139 backport

[openedx-webhooks]

Thanks for the pull request, @mariajgrimaldi!

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