SPARK-826: fold(), reduce(), collect() always attempt to use java serialization

[SPARK-826](https://spark-project.atlassian.net/browse/SPARK-826): "fold(), reduce(), collect() always attempt to use java serialization"

Tested with local and standalone mode only at this point. 
But i think it should work since i patched all existing implementations of TaskSetManager, my guess is  ClusterTaskSetManager is used with Mesos backend as well. 

I will have to test it with mesos anyway soon since this is our deployment mode.