Patch how mongoid tracks Array field changes.

https://www.pivotaltracker.com/story/show/63702868

Mongoid tracking of Array field changes doesn't work as expected
in 2.x versions. Accessing the fields will mark them as dirty.
See mongoid/mongoid#2311 for details.

The problem can be expressed with the following snippet:
```
bs = BusinessSupportEdition.published.last
bs.changes # => {}
bs.sectors # => [] # Accessing an Array field marks it as a change.
bs.changes # => {:sectors=>nil}
```