[SPARK-4130][MLlib] Fixing libSVM parser bug with extra whitespace

This simple patch filters out extra whitespace entries.

Author: Joseph E. Gonzalez <joseph.e.gonzalez@gmail.com>
Author: Joey <joseph.e.gonzalez@gmail.com>

Closes apache#2996 from jegonzal/loadLibSVM and squashes the following commits:

e0227ab [Joey] improving readability
e028e84 [Joseph E. Gonzalez] fixing whitespace bug in loadLibSVMFile when parsing libSVM files

mllib/src/main/scala/org/apache/spark/mllib/util/MLUtils.scala

@@ -76,7 +76,7 @@ object MLUtils {
       .map { line =>
         val items = line.split(' ')
         val label = items.head.toDouble
-        val (indices, values) = items.tail.map { item =>
+        val (indices, values) = items.tail.filter(_.nonEmpty).map { item =>
           val indexAndValue = item.split(':')
           val index = indexAndValue(0).toInt - 1 // Convert 1-based indices to 0-based.
           val value = indexAndValue(1).toDouble