[SPARK-11793][SQL] Dataset should set the resolved encoders internally for maps.

I also wrote a test case -- but unfortunately the test case is not working due to SPARK-11795.

Author: Reynold Xin <rxin@databricks.com>

Closes #9784 from rxin/SPARK-11503.

sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala

@@ -199,11 +199,12 @@ class Dataset[T] private[sql](
    * @since 1.6.0
    */
   def mapPartitions[U : Encoder](func: Iterator[T] => Iterator[U]): Dataset[U] = {
+    encoderFor[T].assertUnresolved()
     new Dataset[U](
       sqlContext,
       MapPartitions[T, U](
         func,
-        encoderFor[T],
+        resolvedTEncoder,
         encoderFor[U],
         encoderFor[U].schema.toAttributes,
         logicalPlan))

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

@@ -73,6 +73,17 @@ class DatasetSuite extends QueryTest with SharedSQLContext {
       ("a", 2), ("b", 3), ("c", 4))
   }
 
+  ignore("Dataset should set the resolved encoders internally for maps") {
+    // TODO: Enable this once we fix SPARK-11793.
+    val ds: Dataset[(ClassData, Long)] = Seq(ClassData("one", 1), ClassData("two", 2)).toDS()
+        .map(c => ClassData(c.a, c.b + 1))
+        .groupBy(p => p).count()
+
+    checkAnswer(
+      ds,
+      (ClassData("one", 1), 1L), (ClassData("two", 2), 1L))
+  }
+
   test("select") {
     val ds = Seq(("a", 1) , ("b", 2), ("c", 3)).toDS()
     checkAnswer(