pd.unique doesn't actually return unique elements of SparseArray
#### Code Sample, a copy-pastable example if possible

```python
pd.unique(pd.SparseArray([0,1,2,3], fill_value=3)) #=> array([0,1,2])

pd.unique(pd.Series([0,1,2,3])) #=> array([0,1,2,3])
```
#### Problem description

So I was digging into #5078 and stumbled across this problem.

#### Expected Output

I would expect the two to be the same.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: 36f905285c0089228985cffc9f9f6c7d28789128
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.13.16-202.fc26.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.0.dev0+253.g36f905285
pytest: 3.2.1
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: 0.26.1
numpy: 1.13.3
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.1.0
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.0
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: 2.7.3.2 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
