Cactus flowers cannot be placed on top of farmland

The Bug:

Cactus flowers cannot be placed on top of farmland.

Despite farmland being a dirt-type block which can support short grass, tall grass, dry grass and flowers, it cannot support cactus flowers.

Steps to Reproduce:

1. Place down some farmland.
2. Attempt to place a cactus flower on top of the farmland.

Observed Behavior:

Cactus flowers cannot be placed on top of farmland.

Expected Behavior:

Cactus flowers would be able to be placed on top of farmland.