Change the key for an existing item in the collection. If the old key
does not exist this is a no-op.
@param {Object} oldKey The old key
@param {Object} newKey The new key
function(oldKey, newKey) {
        var me = this,
            map = me.map,
            indexMap = me.indexMap,
            index = me.indexOfKey(oldKey),
            item;
            
        if (index > -1) {
            item = map[oldKey];
            delete map[oldKey];
            delete indexMap[oldKey];
            map[newKey] = item;
            indexMap[newKey] = index;
            me.keys[index] = newKey;
            me.generation++;
            
        }
    }