NODE-1049 only include primary server if there are no secondary serve… (#194)
NODE-1049 only include primary server if there are no secondary serve… (#194)

* NODE-1049 only include primary server if there are no secondary servers for readPrefrence secondaryPreferred

* minor correction to fix test error

* removed commented out code and stray 2

* minor tweak to test case to make less brittle

mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native