[SPARK-16173] [SQL] Can't join describe() of DataFrame in Scala 2.10

## What changes were proposed in this pull request?

This PR fixes `DataFrame.describe()` by forcing materialization to make the `Seq` serializable. Currently, `describe()` of DataFrame throws `Task not serializable` Spark exceptions when joining in Scala 2.10.

## How was this patch tested?

Manual. (After building with Scala 2.10, test on `bin/spark-shell` and `bin/pyspark`.)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13900 from dongjoon-hyun/SPARK-16173.

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

@@ -1908,7 +1908,8 @@ class Dataset[T] private[sql](
     // All columns are string type
     val schema = StructType(
       StructField("summary", StringType) :: outputCols.map(StructField(_, StringType))).toAttributes
-    LocalRelation.fromExternalRows(schema, ret)
+    // `toArray` forces materialization to make the seq serializable
+    LocalRelation.fromExternalRows(schema, ret.toArray.toSeq)
   }
 
   /**