Sets an item onto Map at passed in priority.  If no
priority is passed in value is set at internally incremented
priority.
@param key {*}
@param value {*}
@param priority {Number}
@overrides sjl.stdlib.SjlMap#set
@method sjl.stdlib.PriorityList#set
@returns {sjl.stdlib.PriorityList}
function (key, value, priority) {
            SjlMap.prototype.set.call(this, key, this.resolveItemWrapping(key, value, priority));
            this._sorted = false;
            return this;
        }