function customDefaultsAssignIn(objValue, srcValue, key, object) {
  if (objValue === undefined || eq(objValue, objectProto[key]) && !hasOwnProperty.call(object, key)) {
    return srcValue;
  }

  return objValue;
}
Used by `_.defaults` to customize its `_.assignIn` use to assign properties
of source objects to the destination object for all destination properties
that resolve to `undefined`.

@private
@param {*} objValue The destination value.
@param {*} srcValue The source value.
@param {string} key The key of the property to assign.
@param {Object} object The parent object of `objValue`.
@returns {*} Returns the value to assign.