SPARK-6878 [CORE] Fix for sum on empty RDD fails with exception

Author: Erik van Oosten <evanoosten@ebay.com>

Closes #5489 from erikvanoosten/master and squashes the following commits:

1c91954 [Erik van Oosten] Rewrote double range matcher to an exact equality assert (SPARK-6878)
f1708c9 [Erik van Oosten] Fix for sum on empty RDD fails with exception (SPARK-6878)

core/src/main/scala/org/apache/spark/rdd/DoubleRDDFunctions.scala

@@ -31,7 +31,7 @@ import org.apache.spark.util.StatCounter
 class DoubleRDDFunctions(self: RDD[Double]) extends Logging with Serializable {
   /** Add up the elements in this RDD. */
   def sum(): Double = {
-    self.reduce(_ + _)
+    self.fold(0.0)(_ + _)
   }
 
   /**

core/src/test/scala/org/apache/spark/rdd/DoubleRDDSuite.scala

@@ -22,6 +22,12 @@ import org.scalatest.FunSuite
 import org.apache.spark._
 
 class DoubleRDDSuite extends FunSuite with SharedSparkContext {
+  test("sum") {
+    assert(sc.parallelize(Seq.empty[Double]).sum() === 0.0)
+    assert(sc.parallelize(Seq(1.0)).sum() === 1.0)
+    assert(sc.parallelize(Seq(1.0, 2.0)).sum() === 3.0)
+  }
+
   // Verify tests on the histogram functionality. We test with both evenly
   // and non-evenly spaced buckets as the bucket lookup function changes.
   test("WorksOnEmpty") {