Top and side faces of fences are not culled by blocks that occlude them

The bug

If an opaque block is placed on top of a fence or next to it, the top face of the fencepost is not culled, nor are the edges of the two bars which touch the block.

Note that the bottom face of the fencepost is in fact culled, which is the correct behaviour.

How to reproduce

1. Place blocks next to fences - test all six sides
2. Enter Spectator mode
3. Inspect

Expected results

Faces which can never be seen would be culled.

Actual results

Only the bottom face is culled. The top face and side faces are not.