BUG: read_hdf modifying passed columns list (GH7212)
Closes #7212. If this object is a list, make a copy and then pass to process_axes - otherwise, just pass the object directly (in this case it is most likely none).

A test for this behavior seems complex and somewhat contrived when compared with what exists in test_pytables.py, so I didn't add one --- let me know any thoughts on this.

I know it seems somewhat contrived, but we do try to add tests even for things like this.
 @shoyer thanks for the feedback! much better solution. i'll get a test together as well.
 @ajamian can you add a test for this?
 @jreback absolutely, should be getting to it this weekend.
 @shoyer @jreback thanks for the feedback --- let me know if you have any other comments
 @ajamian one last thing -- can you add a note to the release notes for 0.17? otherwise this looks good to me!
 yep lgtm as well - pls add a comment on the test with the issue number
 I put the note into doc/source/whatsnew/v0.17.0.txt, I think this is where it belongs...and comment added to test. Thanks guys - let me know if you have any more comments.
 Thanks!
 thanks Tom!
