[SPARK-7181] [CORE] fix inifite loop in Externalsorter's mergeWithAgg…

…regation

see [SPARK-7181](https://issues.apache.org/jira/browse/SPARK-7181).

Author: Qiping Li <liqiping1991@gmail.com>

Closes apache#5737 from chouqin/externalsorter and squashes the following commits:

2924b93 [Qiping Li] fix inifite loop in Externalsorter's mergeWithAggregation

core/src/main/scala/org/apache/spark/util/collection/ExternalSorter.scala

@@ -527,7 +527,8 @@ private[spark] class ExternalSorter[K, V, C](
           val k = elem._1
           var c = elem._2
           while (sorted.hasNext && sorted.head._1 == k) {
-            c = mergeCombiners(c, sorted.head._2)
+            val pair = sorted.next()
+            c = mergeCombiners(c, pair._2)
           }
           (k, c)
         }

core/src/test/scala/org/apache/spark/util/collection/ExternalSorterSuite.scala

@@ -506,7 +506,10 @@ class ExternalSorterSuite extends FunSuite with LocalSparkContext with PrivateMe
     val agg = new Aggregator[Int, Int, Int](i => i, (i, j) => i + j, (i, j) => i + j)
     val ord = implicitly[Ordering[Int]]
     val sorter = new ExternalSorter(Some(agg), Some(new HashPartitioner(3)), Some(ord), None)
-    sorter.insertAll((0 until 100000).iterator.map(i => (i / 2, i)))
+
+    // avoid combine before spill
+    sorter.insertAll((0 until 50000).iterator.map(i => (i , 2 * i)))
+    sorter.insertAll((0 until 50000).iterator.map(i => (i, 2 * i + 1)))
     val results = sorter.partitionedIterator.map{case (p, vs) => (p, vs.toSet)}.toSet
     val expected = (0 until 3).map(p => {
       (p, (0 until 50000).map(i => (i, i * 4 + 1)).filter(_._1 % 3 == p).toSet)