Fixed comparisons to show actual value for OBSERV, if it exists

**Story:** https://www.pivotaltracker.com/story/show/63578694

- Initialize an OBSERV expectation of undefined if an actual OBSERV value exists in the corresponding <code>result</code>
- Addresses bug where a patient is passing even when the OBSERV value is cleared and the non-OBSERV populations are matching

Tested with NQF0496, via <code>rake bonnie:db:reset DB=bonnie-production-eh</code>

![actual_observ_values](https://f.cloud.github.com/assets/456952/1924156/73dc6ff0-7e1e-11e3-8d4f-9eda475ee56d.png)