BUG: bug in xs for a Series with a multiindex (GH6258)
closes #6258
closes #5684

I've got a commit ready adding the test from https://github.com/pydata/pandas/issues/5684. It's similar, but uses labels instead of integers for the level, so it may be useful.

For the PR will I submit it against this branch, `jreback/pandas/xs_bug` instead of `pydata/pandas/master`?
 sure
 I think I did that right: https://github.com/jreback/pandas/pull/8
 yep...you did gr8

I merged it into the PR, which I didn't need to do as that commit just shows up on the PR itself. So then rebased it away (and had to add in a release notes entry). but all good.
