Returns the object's keys.
@param {Object} o The object to operate on.
@returns {Array<String|Number>} The object's key set
@function
function keys (o) {
                    if(o === undefined || o === null) return [];

                    var keys = getKeys(o), idx;
                    if(libs.object.isArguments(o)) {
                        idx = keys.indexOf('length');
                        if(idx > -1) keys.splice(idx, 1);
                    }
                    return keys;
                }