Use boss entity as source for obsidian bomb.

Makes the boss entity the source of the obsidian bomb explosion, which
then makes the damage event listener handle the explosion damage as if
the boss is the damager, which means the `monster-infight` flag should
be respected.

Fixes #759

src/main/java/com/garbagemule/MobArena/waves/ability/core/ObsidianBomb.java

@@ -55,7 +55,7 @@ public class ObsidianBomb implements Ability
                     return;
 
                 world.getBlockAt(loc).setType(Material.AIR);
-                world.createExplosion(loc, 3F);
+                world.createExplosion(loc, 3F, false, true, boss.getEntity());
             }
         }, FUSE);
     }