[SPARK-7452] [MLLIB] fix bug in topBykey and update test

the toArray function of the BoundedPriorityQueue does not necessarily preserve order. Add a counter-example as the test, which would fail the original impl.

Author: Shuo Xiang <shuoxiangpub@gmail.com>

Closes #5990 from coderxiang/topbykey-test and squashes the following commits:

98804c9 [Shuo Xiang] fix bug in topBykey and update test

mllib/src/main/scala/org/apache/spark/mllib/rdd/MLPairRDDFunctions.scala

@@ -46,7 +46,7 @@ class MLPairRDDFunctions[K: ClassTag, V: ClassTag](self: RDD[(K, V)]) extends Se
       combOp = (queue1, queue2) => {
         queue1 ++= queue2
       }
-    ).mapValues(_.toArray.reverse)  // This is an min-heap, so we reverse the order.
+    ).mapValues(_.toArray.sorted(ord.reverse))  // This is an min-heap, so we reverse the order.
   }
 }
 

mllib/src/test/scala/org/apache/spark/mllib/rdd/MLPairRDDFunctionsSuite.scala

@@ -24,13 +24,14 @@ import org.apache.spark.mllib.rdd.MLPairRDDFunctions._
 
 class MLPairRDDFunctionsSuite extends FunSuite with MLlibTestSparkContext {
   test("topByKey") {
-    val topMap = sc.parallelize(Array((1, 1), (1, 2), (3, 2), (3, 7), (5, 1), (3, 5)), 2)
-      .topByKey(2)
+    val topMap = sc.parallelize(Array((1, 7), (1, 3), (1, 6), (1, 1), (1, 2), (3, 2), (3, 7), (5,
+      1), (3, 5)), 2)
+      .topByKey(5)
       .collectAsMap()
 
     assert(topMap.size === 3)
-    assert(topMap(1) === Array(2, 1))
-    assert(topMap(3) === Array(7, 5))
+    assert(topMap(1) === Array(7, 6, 3, 2, 1))
+    assert(topMap(3) === Array(7, 5, 2))
     assert(topMap(5) === Array(1))
   }
 }