The sides of sign faces aren't culled by blocks that completely cover them

The Bug:

The sides of sign faces aren't culled by blocks that completely cover them.

Steps to Reproduce:

1. Place down two signs, both next to one another and facing the same direction.
2. Switch to spectator mode and look at the side faces of the signs.
3. Take note as to whether or not the sides of sign faces are culled by blocks that completely cover them.

Observed Behavior:

The sides of sign faces aren't culled by blocks that completely cover them.

Expected Behavior:

The sides of sign faces would be culled by blocks that completely cover them.