[SPARK-6299][CORE] ClassNotFoundException in standalone mode when running groupByKey with class defined in REPL

```
case class ClassA(value: String)
val rdd = sc.parallelize(List(("k1", ClassA("v1")), ("k1", ClassA("v2")) ))
rdd.groupByKey.collect
```
This code used to be throw exception in spark-shell, because while shuffling ```JavaSerializer```uses ```defaultClassLoader``` which was defined like ```env.serializer.setDefaultClassLoader(urlClassLoader)```.

It should be ```env.serializer.setDefaultClassLoader(replClassLoader)```, like
```
    override def run() {
      val deserializeStartTime = System.currentTimeMillis()
      Thread.currentThread.setContextClassLoader(replClassLoader)
```
in TaskRunner.

When ```replClassLoader``` cannot be defined, it's identical with ```urlClassLoader```

Author: Kevin (Sangwoo) Kim <sangwookim.me@gmail.com>

Closes #5046 from swkimme/master and squashes the following commits:

fa2b9ee [Kevin (Sangwoo) Kim] stylish test codes ( collect -> collect() )
6e9620b [Kevin (Sangwoo) Kim] stylish test codes ( collect -> collect() )
d23e4e2 [Kevin (Sangwoo) Kim] stylish test codes ( collect -> collect() )
a4a3c8a [Kevin (Sangwoo) Kim] add 'class defined in repl - shuffle' test to ReplSuite
bd00da5 [Kevin (Sangwoo) Kim] add 'class defined in repl - shuffle' test to ReplSuite
c1b1fc7 [Kevin (Sangwoo) Kim] use REPL class loader for executor's serializer