Slimes and Magma Cubes ignore leads

Neither slimes nor magma cubes follow typical lead/leashing behavior. When tied to a fence, the slime/magma cube will hop continuously in a single direction, extending the "rope" texture of the lead seemingly infinitely. When tied to other mobs, both slimes and magma cubes will ignore the leash owner, and can break the lead by moving too far out of the leash owner's range.