SPARK-2077 Log serializer that actually ends up being used

I could settle with this being a debug also if we provided an example of how to turn it on in `log4j.properties`

https://issues.apache.org/jira/browse/SPARK-2077

Author: Andrew Ash <andrew@andrewash.com>

Closes #1017 from ash211/SPARK-2077 and squashes the following commits:

580f680 [Andrew Ash] Drop to debug
0266415 [Andrew Ash] SPARK-2077 Log serializer that actually ends up being used

core/src/main/scala/org/apache/spark/SparkEnv.scala

@@ -183,6 +183,7 @@ object SparkEnv extends Logging {
 
     val serializer = instantiateClass[Serializer](
       "spark.serializer", "org.apache.spark.serializer.JavaSerializer")
+    logDebug(s"Using serializer: ${serializer.getClass}")
 
     val closureSerializer = instantiateClass[Serializer](
       "spark.closure.serializer", "org.apache.spark.serializer.JavaSerializer")