Let observers create after_find if they need it

Ticket #625 removed the line that caused observers to define an after_find method in the model if it didn't exist yet. This caused a speed-up (defining after_find can slow things down), but for cases where after_find is observed and not defined in the model, it means that an empty after_find must be explicitly defined in the model. It can be tricky to work out why an observer is failing due to this.

This patch restores the previous behavior, but only if the observer actually has an after_find defined. Hopefully, this offers the best of both worlds.