BUG: array_equivalent was not passing its own doctest. 
The examples in the docstring showed lists being passed to array_equivalent. `array_equivalent` expects `ndarrays` only. (It raises an `AttributeError` when passed a list.)

Also, the use of `pd.isnull` can be removed from `array_equivalent`. My thinking here was muddled. `pd.isnull` is only needed if `array_equivalent` were to accept objects other than ndarrays. `np.array_equal` works as desired for all `ndarrays` except those of float or complex dtype.
