Iron Golem Breaks Lead

An Iron golem tied to a post will break the lead to attack a hostile mob.

What I expected:
tying an iron golem to a post so it doesn't wander off and get lost

What happened:
The Iron golem broke the lead to reach a spider just out of the leads reach.

How to recreate:
Tie an iron golem to a post, put a hostile mob next to the golem, and the iron golem breaks the lead.

Tested only in 14w04b.
Edit-14w05b and 14w06b now tested and confirmed