Update NetworkClient to set Accept header to application/json

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Update our NetworkClient class to add an explicit Accept https://github.com/learningequality/kolibri/blob/develop/kolibri/core/discovery/utils/network/client.py#L50|header of application/json as per: https://www.django-rest-framework.org/api-guide/content-negotiation/#determining-the-accepted-renderer

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