Fixed the weapon to be of the assailant, not the victim.

I noticed when using `.getWeapon()` on an instance of `MeleeDamageInfo`, the returned weapon material was the item that the victim was holding, not the assailant. As per this comment in `MeleeDamageInfo.java`;

```java
/**
 * Gets the material of the weapon that the assailant used.
 *
 * Note: fist kills will return {@link Material.AIR}.
 *
 * @return Material of weapon used
 */
@Nonnull Material getWeapon();
```

I determined that this was a possible bug and made the change.