Get and return the value of the filter.
@return {String} The value of this filter
function () {
        var result = {},
            fields = this.fields, 
            key, field;
            
        for (key in fields) {
            if (fields.hasOwnProperty(key)) {
                field = fields[key];
                if (field.isValid() && field.getValue() !== null) {
                    result[key] = field.getValue();
                }
            }
        }
        return result;
    }