[BUG] Series unique should return an array rather than a dataframe

Pandas's unique method returns an array, while ours returns a Series.

```python
import cudf
print(cudf.__version__)
​
df = cudf.DataFrame({
"a": [0,1,2,0,1,2,0]
})
type(df['a'].unique()), type(df['a'].to_pandas().unique())
0.20.0a+248.g7623f3978b
(cudf.core.series.Series, numpy.ndarray)
```