@Override
    public Set<String> fieldKeys() {
        return new FieldSet(this);
    }
Returns the keys of the fields of this hit as a modifiable view.
This follows the rules of key sets returned from maps: Key removals are reflected
in the map, add and addAll is not supported.
container-search/src/main/java/com/yahoo/prelude/fastsearch/FastHit.java