Add failing example for a model with callback
Add failing example for a model with callback

A model that calls update_attributes from a callack triggers
a second pass through our callback chain and results in
some values being mutated twice which leads to doubly incremented
position values.

corwinstephen/acts_as_list,amitree/acts_as_list,botandrose/acts_as_list,swanandp/acts_as_list,tinderfields/acts_as_list