Moss can spread outside of the world border

The Bug:

Moss can spread outside of the world border.

Steps to Reproduce:

1. Locate the edge of the world border and place some moss along the edge of it.
2. Apply some bone meal to the moss.
3. Take note as to whether or not moss can spread outside of the world border.

Observed Behavior:

Moss can spread outside of the world border.

Expected Behavior:

Moss would not be able to spread outside of the world border.