In the current implementation it was possible for the reference to change after analysis.

Author: Michael Armbrust <michael@databricks.com>

Closes #2717 from marmbrus/pythonUdfResults and squashes the following commits:

da14879 [Michael Armbrust] Fix test
6343bcb [Michael Armbrust] add test
9533286 [Michael Armbrust] Correctly preserve the result attribute of python UDFs though transformations