BUG: array_equivalent was not passing its own doctest. 

The examples showed lists being passed to array_equivalent. array_equivalent expects ndarrays only.

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 object other than ndarrays. `np.array_equal` works as desired for all ndarrays except those of float or complex dtype.