Fix deserialization of JavaSerializer instances.

This caused a failure in a new test; this problem
occurs when calls ShuffledRDD.setSerializer() with
a JavaSerializer.

core/src/main/scala/org/apache/spark/serializer/JavaSerializer.scala

@@ -121,6 +121,8 @@ class JavaSerializer(conf: SparkConf) extends Serializer with Externalizable {
   private var counterReset = conf.getInt("spark.serializer.objectStreamReset", 100)
   private var extraDebugInfo = conf.getBoolean("spark.serializer.extraDebugInfo", true)
 
+  protected def this() = this(new SparkConf())  // For deserialization only
+
   override def newInstance(): SerializerInstance = {
     val classLoader = defaultClassLoader.getOrElse(Thread.currentThread.getContextClassLoader)
     new JavaSerializerInstance(counterReset, extraDebugInfo, classLoader)

core/src/test/scala/org/apache/spark/serializer/JavaSerializerSuite.scala

@@ -0,0 +1,29 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.serializer
+
+import org.apache.spark.SparkConf
+import org.scalatest.FunSuite
+
+class JavaSerializerSuite extends FunSuite {
+  test("JavaSerializer instances are serializable") {
+    val serializer = new JavaSerializer(new SparkConf())
+    val instance = serializer.newInstance()
+    instance.deserialize[JavaSerializer](instance.serialize(serializer))
+  }
+}