How to check for unknown/mistyped query parameters?

The current release of servant does not error out (with BadRequest or similar), if there is an unknown parameter passed. For example:

If the following example, the first API call returns sales info for a particular account in a particular country and the second API call returns sales for a particular account (across all countries).

```
GET /analyze/sales?account_id=1&country_iso=US
GET /analyze/sales?account_id=1
-- mistyped version:
GET /analyze/sales?account_id=1&country=US
```

If the country parameter is mistyped, the user gets a response (but the API returns results for the second API call above), but it is the wrong one.

With `servant` the API is neatly modelled in a handler that accepts a `Maybe AccountId` and a `Maybe CountryIso`, however, it is not possible to identify an error condition from an unknown query parameter. Is there a better way to design this API, or a workaround to this problem?