Updated the RoundRobinRoutingServiceTest from JUnit to Spock.
Added some extra items to the setup initialization that exercises a few more lines of the package.
Added a new test to document that a NullPointerException is thrown instead of an IllegalArgumentException as one might think when the SystemModel has a null Node in a Cluster.
Merge pull request #1051 from rackerlabs/REP-829

Updated the RoundRobinRoutingServiceTest from JUnit to Spock.