Fix healing damage classes and damageable serialization and add test (#2727)

* Fix healing damage classes and damageable serialization and add test

* The fall of an empire

* Fix healPerType being -1 instead of 1

Content.Shared/Damage/DamageType.cs

@@ -58,14 +58,16 @@ namespace Content.Shared.Damage
             var classes = DamageClassExtensions.ToDictionary();
 
             foreach (var @class in classes.Keys.ToList())
-            foreach (var type in @class.ToTypes())
             {
-                if (!types.TryGetValue(type, out var damage))
+                foreach (var type in @class.ToTypes())
                 {
-                    continue;
-                }
+                    if (!types.TryGetValue(type, out var damage))
+                    {
+                        continue;
+                    }
 
-                classes[@class] += damage;
+                    classes[@class] += damage;
+                }
             }
 
             return classes;