[SPARK-6858][SQL] Register Java HashMap for SparkSqlSerializer

Since now kyro serializer is used for `GeneralHashedRelation` whether kyro is enabled or not, it is better to register Java `HashMap` in `SparkSqlSerializer`.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #5465 from viirya/register_hashmap and squashes the following commits:

9062601 [Liang-Chi Hsieh] Register Java HashMap for SparkSqlSerializer.

sql/core/src/main/scala/org/apache/spark/sql/execution/SparkSqlSerializer.scala

@@ -18,6 +18,7 @@
 package org.apache.spark.sql.execution
 
 import java.nio.ByteBuffer
+import java.util.{HashMap => JavaHashMap}
 
 import org.apache.spark.sql.types.Decimal
 
@@ -54,6 +55,7 @@ private[sql] class SparkSqlSerializer(conf: SparkConf) extends KryoSerializer(co
     kryo.register(classOf[org.apache.spark.util.collection.OpenHashSet[_]],
                   new OpenHashSetSerializer)
     kryo.register(classOf[Decimal])
+    kryo.register(classOf[JavaHashMap[_, _]])
 
     kryo.setReferences(false)
     kryo