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.
