As of update 54:
"There is a new Gameplay option: Force Heals to Self when Targeting Undead (default: false). While true, when you have an enemy Undead targeted, any positive healing actions will be cast on you instead of the undead."
However sadly negative healing got forgotten. There is no similar option to force negative healing spells to always target self.
Btw a 'target self' toggle, similar to metamagics (that same way could be turned on/off on individual healing spells on quick bar) would be a perfect sollution (instead of a well hidden / easily forgotten gameplay option).
This could be an auto grant feat for every class that can pos/neg heal.