Kelp cannot be placed on soul sand

Summary

Kelp cannot be placed on submerged soul sand blocks. This is important for making bubble columns easily.

 

Steps to reproduce:

1. Create a one block wide column of flowing water (only one source block at the top)
2. Put a soul sand block on the ground where the water flow ends
3. Try and put kelp on it

Observed Results:

Kelp does not get placed

Expected results:

Kelp should get placed