``np.unique`` now returns single ``NaN``
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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.