Validator does not flag additional fields as invalid

[josee-sabourin]

Certain feeds with additional non-GBFS fields are being checked as valid.
Example: Citibike includes fields in station_information.json like electric_bike_surcharge_waiver and external_id, but the validation report says no errors.

I believe this might actually be an issue with the schemas and not the validator itself. I am opening the issue on both repositories to be sure.

My initial instinct for a solution would be to add an additionalProperties: false to the schemas, but would like to make sure before opening a PR.

[richfab]

Duplicate of #50