NSCopying copyWithZone: implementation does not check isSet

The NSCopying copyWithZone: implementation does not check isSet. This causes the copy not to be a true copy, since all the isSet properties will be set to YES on the newly copied object.

Note, updating to provide some more clarity:

In this example, it's fixed:

- (instancetype) copyWithZone:(NSZone *)zone
{
  Foo * val = [Foo new];
  if (_nameIsSet)
  {
    val.name = [self.name copy];
  }
  return val;
}

Without the if check on _nameIsSet, the newly copied object will have _nameIsSet set to YES but with a nil name.

- (void) setName: (NSString *) name {
  _name = name;
  _nameIsSet = YES;
}