Fix a bug where strict json decoder is not used.

[Fethbita]

A json decoder is created however it is not used. The Strict value is essentially useless. This PR fixes the problem and uses the created json decoder and Strict value is also used.

[jamietanna]

Thanks! Great spot.

Oddly enough, it looks like this code path isn't used, as the following change doesn't break any tests 🙃:

 func jsonHandler(_ string, r io.Reader, obj interface{}, strict bool) error {
        d := json.NewDecoder(r)
        if strict {
+               panic("hi")
                d.DisallowUnknownFields()
        }
        return json.NewDecoder(r).Decode(obj)

Worth us investigating further, but in the meantime this is a good solution!