Fix isRdd() to properly check for class
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")

liyuqi/SparkR-pkg,kuch4r/SparkR-pkg,7c00/SparkR-pkg,kuch4r/SparkR-pkg,kuch4r/SparkR-pkg,rickyHong/sparkR-dist,kcompher/SparkR-pkg,amplab-extras/SparkR-pkg,Liziyao/SparkR-pkg,amplab-extras/SparkR-pkg,liyuqi/SparkR-pkg,EugenePig/SparkR-pkg,RevolutionAnalytics/SparkR-pkg,xzw0005/SparkR-pkg,rickyHong/sparkR-dist,7c00/SparkR-pkg,kcompher/SparkR-pkg,LeoYao/SparkR-pkg,EDMunplugged/SparkR-pkg,Draki/SparkR-pkg,xzw0005/SparkR-pkg,nvoron23/SparkR-pkg,7c00/SparkR-pkg,kimchitsigai/SparkR-pkg,EugenePig/SparkR-pkg,kimchitsigai/SparkR-pkg,nvoron23/SparkR-pkg,Liziyao/SparkR-pkg,LeoYao/SparkR-pkg,Draki/SparkR-pkg,liyuqi/SparkR-pkg,nvoron23/SparkR-pkg,EugenePig/SparkR-pkg,xzw0005/SparkR-pkg,EDMunplugged/SparkR-pkg,RevolutionAnalytics/SparkR-pkg,Liziyao/SparkR-pkg,RevolutionAnalytics/SparkR-pkg,rickyHong/sparkR-dist,liyuqi/SparkR-pkg,LeoYao/SparkR-pkg,rickyHong/sparkR-dist,EDMunplugged/SparkR-pkg,nvoron23/SparkR-pkg,RevolutionAnalytics/SparkR-pkg,xzw0005/SparkR-pkg,Liziyao/SparkR-pkg,kimchitsigai/SparkR-pkg,amplab-extras/SparkR-pkg,kuch4r/SparkR-pkg,kcompher/SparkR-pkg,Draki/SparkR-pkg,7c00/SparkR-pkg,LeoYao/SparkR-pkg,EDMunplugged/SparkR-pkg,amplab-extras/SparkR-pkg,kcompher/SparkR-pkg