Performing a mongo $set: {} will fail on mongo 2.6+: https://jira.mongodb.org/browse/SERVER-12266

For now fix this only for the failing test case as it should not be an issue in the field for other
locations.  Long term this should be fixed by the move to mongoengine.
Merge branch 'mongo-behavior-change' into 2.5-testing
Merge branch '2.5-testing' into 2.6-dev
Merge branch '2.6-dev'