Resource states are readonly.

@vcekov @shajith @jamesarosen @ebryn

In Ember 1.x the arity of a computed property function is checked. If it takes no arguments, it can be totally overwritten. E.g. a call to `set` will cause the property to *never* be computed again.

However if you specify that you would like to accept two arguments `key`, and `value`, then the behavior is similar to that of `Ember 0.9.x`.