@Override
        public IntegerFieldValue get(Object key) {
            Object o = map.get(unwrap(key));
            return o == null ? null : wrapValue(o);
        }
Weighted set MapFieldValue, backed by map of native Java types.
Note: The key type of this is FieldValue, not K.
@author vegardh
document/src/main/java/com/yahoo/document/datatypes/WeightedSet.java