BUG: revert assert_numpy_array_equal to before c2ea8fb2 (accept non-ndarrays)
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

`assert_numpy_array_equal`'s docstring is at odds with (and the existence of the `obj` argument clashes with) the fact that it's now strict on its arguments being ndrarrays.

There are [practical reasons](https://github.com/pydata/pandas/pull/13205#discussion_r65358471) to allow other kinds of arguments.

On the other hand, I don't see any reason for being strict, but I assume @sinhrks 's [PR](https://github.com/pydata/pandas/pull/13311) had some rationale behind: so a possible solution is an argument `ensure_ndarray=True` which restores the previous behaviour when set to `False`.
