Checks if a defined value exists for key (or namespace key) and returns it.
If no value is found (key/ns-key is undefined) then returns undefined.
@method sjl.stdlib.Config#has
@param keyOrNsString {String}
@returns {*|undefined}
function (keyOrNsString/*, type{String|Function}...*/) {
                sjl.throwTypeErrorIfNotOfType(contextName + '.has', 'keyOrNsString', keyOrNsString, String);
                var searchResult = sjl.searchObj(keyOrNsString, this);
                return arguments.length === 1 ?
                    sjl.isset(searchResult) :
                    sjl.issetAndOfType.apply(sjl, [searchResult].concat(sjl.restArgs(1)));
            }