Handle unset collection columns persisted records

On master accessing a collection (list, set or map) property of persisted record falls into an infinitely loop if the property had not previously been explicitly set. For example (using the Post model from properties_spec.rb)

```ruby
Post.create! permalink: 'cequel'
Post['cequel'].tags.first #=> raises SystemStackError
```

This is because `Collection#__getobj__` calls `model.write_attribute` which, due to dirty attribute tracking, ends up getting the collection attribute's value which causes another call to `#__getobj__` thus continuing the cycle.