[SPARK-20125][SQL] Dataset of type option of map does not work

## What changes were proposed in this pull request?

When we build the deserializer expression for map type, we will use `StaticInvoke` to call `ArrayBasedMapData.toScalaMap`, and declare the return type as `scala.collection.immutable.Map`. If the map is inside an Option, we will wrap this `StaticInvoke` with `WrapOption`, which requires the input to be `scala.collect.Map`. Ideally this should be fine, as `scala.collection.immutable.Map` extends `scala.collect.Map`, but our `ObjectType` is too strict about this, this PR fixes it.

## How was this patch tested?

new regression test

Author: Wenchen Fan <wenchen@databricks.com>

Closes #17454 from cloud-fan/map.

sql/catalyst/src/main/scala/org/apache/spark/sql/types/ObjectType.scala

@@ -44,4 +44,9 @@ case class ObjectType(cls: Class[_]) extends DataType {
   def asNullable: DataType = this
 
   override def simpleString: String = cls.getName
+
+  override def acceptsType(other: DataType): Boolean = other match {
+    case ObjectType(otherCls) => cls.isAssignableFrom(otherCls)
+    case _ => false
+  }
 }

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

@@ -1154,10 +1154,16 @@ class DatasetSuite extends QueryTest with SharedSQLContext {
     assert(errMsg3.getMessage.startsWith("cannot have circular references in class, but got the " +
       "circular reference of class"))
   }
+
+  test("SPARK-20125: option of map") {
+    val ds = Seq(WithMapInOption(Some(Map(1 -> 1)))).toDS()
+    checkDataset(ds, WithMapInOption(Some(Map(1 -> 1))))
+  }
 }
 
 case class WithImmutableMap(id: String, map_test: scala.collection.immutable.Map[Long, String])
 case class WithMap(id: String, map_test: scala.collection.Map[Long, String])
+case class WithMapInOption(m: Option[scala.collection.Map[Int, Int]])
 
 case class Generic[T](id: T, value: Double)