Thornmail should reflect Magic on-hit item effects

Had to make a new thread to clarify. I think Thornmail should reflect all or part of the damage from magic-damage on-hit items.

It'd be the only counter item to the full hybrid on-hit builds that deal all three damage types, and the inconsistency between it granting armor but having a bonus effect against magic on-hits won't make it overpowered against other auto-attacking champs or Nashor's builders.

Specifically, I don't want it to reflect damage from bork or bloodrazor, to keep it fair against champs who are just building those instead of hybrid damage.