When np.unique operated on an array with multiple NaN entries,
its return included a NaN for each entry that was NaN in the original array.
This is now improved such that the returned array contains just one NaN as the
last element.
Also for complex arrays all NaN values are considered equivalent
(no matter whether the NaN is in the real or imaginary part). As the
representant for the returned array the smallest one in the
lexicographical order is chosen - see np.sort for how the lexicographical
order is defined for complex arrays.