Fix isRdd() to properly check for class

The old approach, class(obj) == "RDD" returns a vector as long
as class attribute of obj.  For example:
> obj <- structure(list(), class = c("classA", "classB"))
> class(obj) == "RDD"
[1] FALSE FALSE
This causes getDependencies to fail.  Changed to:
inherits(obj, "RDD")