@Override
        public IntegerFieldValue put(FieldValue key, IntegerFieldValue value) {
            IntegerFieldValue old = get(key);
            map.put(unwrap(key), (Integer) unwrap(value));
            return old;
        }
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