Sets a key-value pair in this instance.
@method sjl.stdlib.SjlMap#set
@param key {*} - Key to set.
@param value {*} - Value to set.
@returns {SjlMap}
function (key, value) {
            var index = this._keys.indexOf(key);
            if (index > -1) {
                this._keys[index] = key;
                this._values[index] = value;
            }
            else {
                this._keys.push(key);
                this._values.push(value);
            }
            return this;
        }