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 think we don't need a key tag for writable. We only need key tag for value type (i.e. ClassTag.Int, etc.). This is not used anywhere. I suspect just matching on the class should be o.k.