Update JsonProtocol test, make property name consistent.

Author: Marcelo Vanzin

core/src/main/scala/org/apache/spark/util/JsonProtocol.scala

@@ -195,7 +195,7 @@ private[spark] object JsonProtocol {
     ("App ID" -> applicationStart.appId.map(JString(_)).getOrElse(JNothing)) ~
     ("Timestamp" -> applicationStart.time) ~
     ("User" -> applicationStart.sparkUser) ~
-    ("appAttemptId" -> applicationStart.appAttemptId.map(JString(_)).getOrElse(JNothing))
+    ("App Attempt ID" -> applicationStart.appAttemptId.map(JString(_)).getOrElse(JNothing))
   }
 
   def applicationEndToJson(applicationEnd: SparkListenerApplicationEnd): JValue = {

core/src/test/scala/org/apache/spark/util/JsonProtocolSuite.scala

@@ -275,9 +275,11 @@ class JsonProtocolSuite extends FunSuite {
 
   test("SparkListenerApplicationStart backwards compatibility") {
     // SparkListenerApplicationStart in Spark 1.0.0 do not have an "appId" property.
+    // SparkListenerApplicationStart pre-Spark 1.4 does not have "appAttemptId".
     val applicationStart = SparkListenerApplicationStart("test", None, 1L, "user", None)
     val oldEvent = JsonProtocol.applicationStartToJson(applicationStart)
       .removeField({ _._1 == "App ID" })
+      .removeField({ _._1 == "App Attempt ID" })
     assert(applicationStart === JsonProtocol.applicationStartFromJson(oldEvent))
   }