[heal_unit]: ensure that heal amount is an integer

(cherry-picked from commit 76ed49a)

data/lua/wml/heal_unit.lua

@@ -31,7 +31,7 @@ function wesnoth.wml_actions.heal_unit(cfg)
 			new_hitpoints = u.max_hitpoints
 		else
 			heal_amount = tonumber(cfg.amount) or heal_amount
-			new_hitpoints = math.max(1, math.min(u.max_hitpoints, u.hitpoints + heal_amount))
+			new_hitpoints = math.floor(math.max(1, math.min(u.max_hitpoints, u.hitpoints + heal_amount)))
 			heal_amount = new_hitpoints - u.hitpoints
 		end