add unit test for null queries
add unit test for null queries

https://github.com/foursquare/fongo/pull/26
http://stackoverflow.com/questions/12403240/storing-null-vs-not-storing-the-key-at-all-in-mongodb

bwaldvogel/mongo-java-server,bwaldvogel/mongo-java-server,bwaldvogel/mongo-java-server