[SPARK-10591][SQL][TEST] Add a testcase to ensure if checkAnswer handles map correctly

## What changes were proposed in this pull request?

This PR adds a testcase to ensure if `checkAnswer` handles Map type correctly.

## How was this patch tested?

Pass the jenkins tests.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13913 from dongjoon-hyun/SPARK-10591.

sql/core/src/test/scala/org/apache/spark/sql/DatasetSuite.scala

@@ -34,6 +34,13 @@ class DatasetSuite extends QueryTest with SharedSQLContext {
 
   private implicit val ordering = Ordering.by((c: ClassData) => c.a -> c.b)
 
+  test("checkAnswer should compare map correctly") {
+    val data = Seq((1, "2", Map(1 -> 2, 2 -> 1)))
+    checkAnswer(
+      data.toDF(),
+      Seq(Row(1, "2", Map(2 -> 1, 1 -> 2))))
+  }
+
   test("toDS") {
     val data = Seq(("a", 1), ("b", 2), ("c", 3))
     checkDataset(