Make Model.validate() use castObject() to cast, and also not modify the object in place

[vkarpov15]

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Issue

Model.castObject(obj) casts obj and returns a copy if casting succeeded. Model.validate(obj) casts and validates obj, but modifies obj in place if casting succeeded.

This behavior is inconsistent, and I think we should either (1) make validate() not run casting, or (2) make validate() return a casted copy. The major reason for returning a copy instead of modifying the object in place is to avoid only applying casting halfway if casting fails. If there's a cast error, some of the paths in obj won't be casted.