API xgrids function doesn't dinamically load the class of the new gridlines

* **C3 version**: 0.4.18
* **D3 version**: 3.5.17
* **Browser**: Chrome Version 61.0.3163.100 (Official Build) (64-bit)
* **OS**: Windows 10 64-bit

Hi, I've been using this library for a while and it's really good, congrats.
Recently I have found a problem when loading a new set of x grid lines which have different classes.
Please check out this fiddle: http://jsfiddle.net/hm5znbgt/1/
- in the first step two grid lines are loaded one having the class "active" and the other one beeing "inactive"
- then in the seconds step a new grid line is loaded having the class "inactive", but the grid is loaded with the "active" class for some reason

Thanks,
Gabor