JSON endpoint can return XML

[PGijsbers]

Querying https://www.openml.org/api/v1/json/data/tag?data_id=2&tag=test&api_key=000 results in an error in XML, instead of JSON. Note that this is not the case for all errors, e.g. https://www.openml.org/api/v1/json/data/24567890 shows the error in JSON format.