Add Poison Immunity Bypass to Mythical Beast

[gdsdougherty]

The vanilla pets that deal poison damage (monitor lizard and centipede) are effectively worse than their standard counterparts since they're balanced around the fact that they do poison damage, in a game where 90% of enemies are immune to poison. It seems like an oversight that taking the vanilla mythical beast mythic ability doesn't ignore poison immunities, so it would be nice to have.