This is a workaround for a problem reported on the user list.  This doesn't fix the core problem, but in general is a more robust way to do renames.

Author: Michael Armbrust <michael@databricks.com>

Closes #5337 from marmbrus/toDFrename and squashes the following commits:

6a3159d [Michael Armbrust] [SPARK-6686][SQL] Use resolved output instead of names for toDF rename