Use correct MIME type for JSON response (if possible)

Client libraries and browsers may offer better handling of JSON response content, if they know it's json. In case it's easy for the server to know which content type it's delivering, it should do so.

One particular case would be in kerchunk.py. The branch which handles metadata requests should return the metadata content with media_type="application/json". I did not yet check if more such instances exist.