code: function copy(target, obj) {
    if (isObject(obj)) {
      forIn(obj, function(value, key) {
        target[key] = value;
      });
    }
  }
docs: copy properties from the source object to the
  target object. We don't use `Object.keys` here, since
  "mixin" also adds non-enumerable keys.
  
  @param  {*} `value`
  @param  {String} `key`