[device] websocket token returns mimetype and or wrong encoding

[jonau]

Python clients responds with message='Attempt to decode JSON with unexpected mimetype: text/html; charset=utf-8', url=URL('http://localhost/devices/ef416742-5c7c-42e4-8176-df8a7bf051eb/websocket')

According to the API the response should be:

    200:
      description: The newly generated websocket token for the device.
      content:
        application/json:
          schema:
            type: string

[jonau]

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