Prevent ContextClassLoader of Actor from becoming ClassLoader of Executo...

...r.

Constructor of `org.apache.spark.executor.Executor` should not set context class loader of current thread, which is backend Actor's thread.

Run the following code in local-mode REPL.

```
scala> case class Foo(i: Int)
scala> val ret = sc.parallelize((1 to 100).map(Foo), 10).collect
```

This causes errors as follows:

```
ERROR actor.OneForOneStrategy: [L$line5.$read$$iwC$$iwC$$iwC$$iwC$Foo;
java.lang.ArrayStoreException: [L$line5.$read$$iwC$$iwC$$iwC$$iwC$Foo;
     at scala.runtime.ScalaRunTime$.array_update(ScalaRunTime.scala:88)
     at org.apache.spark.SparkContext$$anonfun$runJob$3.apply(SparkContext.scala:870)
     at org.apache.spark.SparkContext$$anonfun$runJob$3.apply(SparkContext.scala:870)
     at org.apache.spark.scheduler.JobWaiter.taskSucceeded(JobWaiter.scala:56)
     at org.apache.spark.scheduler.DAGScheduler.handleTaskCompletion(DAGScheduler.scala:859)
     at org.apache.spark.scheduler.DAGScheduler.processEvent(DAGScheduler.scala:616)
     at org.apache.spark.scheduler.DAGScheduler$$anonfun$start$1$$anon$2$$anonfun$receive$1.applyOrElse(DAGScheduler.scala:207)
     at akka.actor.ActorCell.receiveMessage(ActorCell.scala:498)
     at akka.actor.ActorCell.invoke(ActorCell.scala:456)
     at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:237)
     at akka.dispatch.Mailbox.run(Mailbox.scala:219)
     at akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(AbstractDispatcher.scala:386)
     at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
     at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
     at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
     at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)
```

This is because the class loaders to deserialize result `Foo` instances might be different from backend Actor's, and the Actor's class loader should be the same as Driver's.