Projectiles don't collide with the world border

The Bug:

Projectiles don't collide with the world border.

This affects all types of projectiles such as arrows, snowballs, eggs, ender pearls, fireballs, etc... This also allows players to get outside of the world border by simply using an ender pearl.

Steps to Reproduce:

1. Set the world border's center to your current position and to twenty blocks wide by using the commands provided below.

   /worldborder center ~ ~

   /worldborder set 20

2. Obtain any projectile, for example, a snowball.
3. Whilst facing the world border, throw/shoot the projectile.
4. Take note as to whether or not projectiles collide with the world border.

Observed Behavior:

Projectiles don't collide with the world border.

Expected Behavior:

Projectiles would collide with the world border.