minor fix

a copy of vertices with defaultVal is created before, and it's b in
(a, b) => b

see in VertexPartition.scala
val fullIter = iter ++ routingTable.iterator.map(vid => (vid, defaultVal))

graphx/src/main/scala/org/apache/spark/graphx/VertexRDD.scala

@@ -392,7 +392,7 @@ object VertexRDD {
    */
   def apply[VD: ClassTag](
       vertices: RDD[(VertexId, VD)], edges: EdgeRDD[_, _], defaultVal: VD): VertexRDD[VD] = {
-    VertexRDD(vertices, edges, defaultVal, (a, b) => b)
+    VertexRDD(vertices, edges, defaultVal, (a, b) => a)
   }
 
   /**