[Django]-Can't change headers with DRF APIClient()

The kwargs passed to the credentials() method ends up feeding directly into the constructor for a WSGIRequest; this means the kwargs it accepts aren’t HTTP headers, but WSGI environment variables. And HTTP headers passed as WSGI env vars are always prefixed with HTTP_ — e.g. the Authorization header is configured with HTTP_AUTHORIZATION. Also, underscores are used in place of dashes.

To have your X-Custom-Header header come out the other side in request.headers (not request.META, which is a copy of the WSGI env vars), pass HTTP_X_CUSTOM_HEADER instead of X-CUSTOM-HEADER.