SC.RecordAttribute Array transform leaks KVO properties to the underlying array

[mauritslamers]

The definition of the Array transform of SC.RecordAttribute (at https://github.com/sproutcore/sproutcore/blob/master/frameworks/datastore/models/record_attribute.js#L454) defines the observesChildren property.
Because of this property, the general toType will add observers to the value (see https://github.com/sproutcore/sproutcore/blob/master/frameworks/datastore/models/record_attribute.js#L233).

Because of the value here being a native array, it results in adding KVO properties to the array. This causes the record hash to be no longer convertible to JSON because of the circular structures the KVO system adds.