[SPARK-6872] [SQL] add copy in external sort

We need add copy before call externalsort.

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #5481 from adrian-wang/extsort and squashes the following commits:

9611586 [Daoyuan Wang] fix bug in external sort

sql/core/src/main/scala/org/apache/spark/sql/execution/basicOperators.scala

@@ -194,7 +194,7 @@ case class ExternalSort(
     child.execute().mapPartitions( { iterator =>
       val ordering = newOrdering(sortOrder, child.output)
       val sorter = new ExternalSorter[Row, Null, Row](ordering = Some(ordering))
-      sorter.insertAll(iterator.map(r => (r, null)))
+      sorter.insertAll(iterator.map(r => (r.copy, null)))
       val baseIterator = sorter.iterator.map(_._1)
       // TODO(marmbrus): The complex type signature below thwarts inference for no reason.
       CompletionIterator[Row, Iterator[Row]](baseIterator, sorter.stop())