fix entity.location.standing_on for slabs

standing on a slab would return the wrong block

src/main/java/net/aufdemrand/denizen/objects/dEntity.java

@@ -1439,7 +1439,7 @@ else if (attribute.startsWith("equipment")) {
         // Returns the location of what the entity is standing on.
         // -->
         if (attribute.startsWith("location.standing_on"))
-            return new dLocation(entity.getLocation().clone().add(0, -1, 0))
+            return new dLocation(entity.getLocation().clone().add(0, -0.5f, 0))
                     .getAttribute(attribute.fulfill(2));
 
         // <--[tag]