fix mistakes: should be able to call with or without mergeFunc

Author: larryxiao

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

@@ -419,7 +419,7 @@ object VertexRDD {
       (vertexIter, routingTableIter) =>
         val routingTable =
           if (routingTableIter.hasNext) routingTableIter.next() else RoutingTablePartition.empty
-        Iterator(ShippableVertexPartition(vertexIter, routingTable, defaultVal))
+        Iterator(ShippableVertexPartition(vertexIter, routingTable, defaultVal, mergeFunc))
     }
     new VertexRDD(vertexPartitions)
   }

graphx/src/main/scala/org/apache/spark/graphx/impl/ShippableVertexPartition.scala

@@ -36,7 +36,16 @@ private[graphx]
 object ShippableVertexPartition {
   /** Construct a `ShippableVertexPartition` from the given vertices without any routing table. */
   def apply[VD: ClassTag](iter: Iterator[(VertexId, VD)]): ShippableVertexPartition[VD] =
-    apply(iter, RoutingTablePartition.empty, null.asInstanceOf[VD])
+    apply(iter, RoutingTablePartition.empty, null.asInstanceOf[VD], (a, b) => a)
+
+  /**
+   * Construct a `ShippableVertexPartition` from the given vertices with the specified routing
+   * table, filling in missing vertices mentioned in the routing table using `defaultVal`.
+   */
+  def apply[VD: ClassTag](
+    iter: Iterator[(VertexId, VD)], routingTable: RoutingTablePartition, defaultVal: VD)
+    : ShippableVertexPartition[VD] = 
+    apply(iter, routingTable, defaultVal, (a, b) => a)
 
   /**
    * Construct a `ShippableVertexPartition` from the given vertices with the specified routing