Core/Unit: Set stand state on damage only to players

Port From (https://github.com/TrinityCore/TrinityCore/commit/730a8da9e99fdb50459e9eb57d79ac995430125e)

Source/Game/Entities/Unit/Unit.cs

@@ -2705,6 +2705,10 @@ namespace Game.Entities
                 }
             }
 
+            // make player victims stand up automatically
+            if (victim.GetStandState() != 0 && victim.IsPlayer())
+                victim.SetStandState(UnitStandStateType.Stand);
+
             return damage;
         }