Make value optional

[yozo1984]

Hi,

First of all, thank you so much for developing this package. It's really useful and do exactly what I expected to do for the most of the time. One minor problem of mine when using the library is that I have to pass in a value. Although it is okay to just leave automatically as undefined, the data will become [undefined] which I found unnecessary. I did some quick modification to stop this. Please feel free to let me know if you think this should not be changed.

Thanks, Zak