.astype('object') misbehaves on Categorical containing Timestamp
#### Code Sample, a copy-pastable example if possible

```python
In [2]: pd._libs.algos.ensure_object([pd.Timestamp('2014-01-01')]) # right
Out[2]: array([Timestamp('2014-01-01 00:00:00')], dtype=object)

In [3]: pd._libs.algos.ensure_object(pd.Categorical([pd.Timestamp('2014-01-01')])) # wrong
Out[3]: array([1388534400000000000], dtype=object)
```
#### Problem description

The two calls should produce the same output. The problem comes from:

``` python
In [3]: pd.Categorical([pd.Timestamp('2014-01-01')]).astype('object')
Out[3]: array([1388534400000000000], dtype=object)
```

... which in turn comes from

``` python
In [3]: np.array(pd.Categorical([pd.Timestamp('2014-01-01')]), dtype='object')
Out[3]: array([1388534400000000000], dtype=object)
```

#### Expected Output

``Out[2]``

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

<details>

INSTALLED VERSIONS
------------------
commit: 4329a8d03555568b8593364caad680c24f91c9ad
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-3-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.22.0.dev0+18.g4329a8d03
pytest: 3.0.6
pip: 9.0.1
setuptools: None
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 5.1.0.dev
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: None
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.6
lxml: None
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1
</details>
