Author: Michael Armbrust <michael@databricks.com>

Closes #6165 from marmbrus/wrongColumn and squashes the following commits:

4fad158 [Michael Armbrust] Merge remote-tracking branch 'origin/master' into wrongColumn
aad7eab [Michael Armbrust] rxins comments
f1e8df1 [Michael Armbrust] [SPARK-6743][SQL] Fix empty projections of cached data