mergeDefined (target, source) {
        var key;

        for (key in source) {
            if (_.has(source, key) && !_.isUndefined(source[key])) {
                target[key] = source[key];
            }
        }

        return target;
    }
Merges defined keys from the target object onto the source object.

@param {Object} target -
@param {Object} source -
@returns {Object}