MAHOUT-1615: drmFromHDFS returning the same Key for all Key,Vec Pairs for Text-Keyed SequenceFiles

SparkContext.sequenceFile(...) will yield the same key per partition for Text-Keyed Sequence files if the key a new copy of the key is not created when mapping to an RDD.  This patch checks for Text Keys and creates a copy of each Key if necessary.    
i mean, this should just be 

     val (key2valFunc, unwrappedKeyTag) = keyTag match {....


and `key2valFunc` should eventually be used for transformation.