Signs can be placed on top of doors

The Bug:

Signs normally require some kind of visual attachment to a supporting block in order to exist in the world, however, they can be placed on top of doors causing them to appear floating.

Steps to Reproduce:

1. Place down a door and obtain a sign.
2. Attempt to place the sign on the top face of the door.
3. Take note as to whether or not signs can be placed on top of doors.

Observed Behavior:

Signs can be placed.

Expected Behavior:

Signs would not be able to be placed on the top of doors.