Support pass reflect value for fill

[maniak89]

I ran into a problem where I don't know the type myself - I get reflect.Value and call Interface(), which returns the type any(Struct{}), but then passing a pointer to a temporary variable storing the result of Interface() to defaults.SetDefaults leads to a panic (panic: reflect: call of reflect.Value.NumField on interface Value).

switch dst.Type().Kind() {
      case reflect.Struct:
        val := dst.Interface()
        defaults.SetDefaults(&val) // <- panic
        dst.Set(reflect.ValueOf(val))

        return val, nil
...