add test

core/src/test/scala/spark/KryoSerializerSuite.scala

@@ -97,6 +97,7 @@ class KryoSerializerSuite extends FunSuite {
     check(mutable.HashMap(1 -> "one", 2 -> "two"))
     check(mutable.HashMap("one" -> 1, "two" -> 2))
     check(List(Some(mutable.HashMap(1->1, 2->2)), None, Some(mutable.HashMap(3->4))))
+    check(List(mutable.HashMap("one" -> 1, "two" -> 2),mutable.HashMap(1->"one",2->"two",3->"three")))
   }
 
   test("custom registrator") {