Spawner upgrades stay; Snakebite poison respects PvP protection

Key changes

• Withdrawing part of an upgraded spawner stack keeps the upgrade levels you paid for, so the spawners you take out no longer come back clean.
• Putting one of those spawners back onto a stack neither charges you twice nor adds a free upgrade level.
• If you are protected from PvP, a Snakebite Bomb no longer poisons you; its other effects are unchanged.