Signs can break solid blocks

If you attach a sign to a partial block, the sign is placed on the side facing you or on top, even if there is a solid block. This bug can be used to break bedrock.

How to Reproduce

1. Place a lower half slab, with a solid block directly above.
2. Place a sign on top of the slab.

Expected Result
The sign should not get placed.

Actual Result
The sign does get placed, deleting the solid block.