Dropper facing other dropper doesn't transfer items if redstone source is on top

The bug

Dropper facing other dropper doesn't transfer items if redstone source is on top

To reproduce

1. Place two droppers facing each other
2. Places some blocks in one dropper
3. Place any redstone source (e.g. lever) on top of the loaded dropper
4. Flip the switch several times -> Target dropper is empty
5. Remove the switch from top and place it besides the loaded dropper
6. Flip the switch several times -> Target dropper has received blocks

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   Demonstration video: http://youtu.be/VNqOA12Kyj8