BUG: Ensure data_columns is always a list (i.e. min_itemsize can exte…
…nd it)

closes #10381

can you rebase/update
 can you rebase / update & add whatsnew for 0.18.2
 can you update according to comments
 can you rebase / update?
 Yeah, I'm just trying to focus on [one PR](https://github.com/pydata/pandas/pull/13205) at a time to limit (my) confusion... but this will be the next.
 sure no prob
 can you rebase / update?
 Not next week, I fear, but yes the one after.
 closing, but pls reopen / comment if you can continue
 See #14728 (sorry for the long wait)