Silk Touch doesn't works with Durability:0 tools

Silk Touch doesn't works when you mine a block with any tool with zero durability and it breaks.
It works before 1.8 and I can confirm it in 1.7.

I made a short video with bug demonstration (sorry, it isn't english but you can see how it works even if it's obvious):

https://www.youtube.com/watch?v=x2elBSbu61c

Regards!