Add weapon type check before infection & Fix nade cooking issues

[Faramour]

PR fixes issues which popped up during tests:

• There's no weapon type check happening before/during infection, meaning that if zombies
  could somehow damage humans with other means than a knife, that damage would infect humans
• When you get infected while cooking a grenade, you continue to hold that grenade after re-spawning as a zombie,
  which bypasses the utility killing inside OnItemEquip and lets zombies throw a grenade
• When you get infected while cooking a grenade, you drop that grenade, which then will hurt humans as the owner of the grenade is now a zombie