Fix issue #105, one-way zero binding.

Adds a zero value check to fix issue #105. Added test showing fix.

Before fix:
![screen shot 2013-06-01 at 10 15 23 am](https://f.cloud.github.com/assets/969390/595012/d1d2c0cc-cace-11e2-84b4-becee966f5ff.png)

After fix:
![screen shot 2013-06-01 at 10 16 14 am](https://f.cloud.github.com/assets/969390/595013/ea9960e8-cace-11e2-9717-21ac85633ebd.png)