One time bindings break when batarang is enabled.

[ryancanulla]

I can reproduce this issue in v10.6.

To reproduce:

1. Create a one-time binding. Test that it does not update when changed with batarang off.
2. Turn on batarang and notice that it now allows the multiple bindings.

Related to #209

[jacek213]

+1, same thing happens for me

https://cloud.githubusercontent.com/assets/3914691/16583963/4a6f003a-42b9-11e6-8031-5e33f56ced46.png

[bxyoung89]

This seems to have been fixed at one point (i.e. #140 ) but I'm still seeing it now.