[SPARK-18009][SQL] Fix ClassCastException while calling toLocalIterator() on dataframe produced by RunnableCommand

## What changes were proposed in this pull request?
A short code snippet that uses toLocalIterator() on a dataframe produced by a RunnableCommand
reproduces the problem. toLocalIterator() is called by thriftserver when
`spark.sql.thriftServer.incrementalCollect`is set to handle queries producing large result
set.

**Before**
```SQL
scala> spark.sql("show databases")
res0: org.apache.spark.sql.DataFrame = [databaseName: string]

scala> res0.toLocalIterator()
16/10/26 03:00:24 ERROR Executor: Exception in task 0.0 in stage 0.0 (TID 0)
java.lang.ClassCastException: org.apache.spark.sql.catalyst.expressions.GenericInternalRow cannot be cast to org.apache.spark.sql.catalyst.expressions.UnsafeRow
```

**After**
```SQL
scala> spark.sql("drop database databases")
res30: org.apache.spark.sql.DataFrame = []

scala> spark.sql("show databases")
res31: org.apache.spark.sql.DataFrame = [databaseName: string]

scala> res31.toLocalIterator().asScala foreach println
[default]
[parquet]
```
## How was this patch tested?
Added a test in DDLSuite

Author: Dilip Biswal <[email protected]>

Closes #15642 from dilipbiswal/SPARK-18009.

Author: dilipbiswal

sql/core/src/main/scala/org/apache/spark/sql/execution/command/commands.scala

@@ -66,6 +66,8 @@ case class ExecutedCommandExec(cmd: RunnableCommand) extends SparkPlan {
 
   override def executeCollect(): Array[InternalRow] = sideEffectResult.toArray
 
+  override def executeToIterator: Iterator[InternalRow] = sideEffectResult.toIterator
+
   override def executeTake(limit: Int): Array[InternalRow] = sideEffectResult.take(limit).toArray
 
   protected override def doExecute(): RDD[InternalRow] = {

sql/core/src/test/scala/org/apache/spark/sql/execution/command/DDLSuite.scala

@@ -1805,4 +1805,11 @@ class DDLSuite extends QueryTest with SharedSQLContext with BeforeAndAfterEach {
       }
     }
   }
+
+  test("SPARK-18009 calling toLocalIterator on commands") {
+    import scala.collection.JavaConverters._
+    val df = sql("show databases")
+    val rows: Seq[Row] = df.toLocalIterator().asScala.toSeq
+    assert(rows.length > 0)
+  }
 }