unstack() on DataFrame of ints with MultiIndex which was sliced casts to float
#### Code Sample, a copy-pastable example if possible

```python
In [2]: idx = pd.MultiIndex.from_product([['a'], ['A', 'B', 'C', 'D']])[:-1]

In [3]: df = pd.DataFrame([[1, 0]]*3, index=idx)

In [4]: df.unstack()
Out[4]: 
     0              1          
     A    B    C    A    B    C
a  1.0  1.0  1.0  0.0  0.0  0.0
```
#### Problem description

Since there is no missing value, no type casting should occur. Notice that even replacing ``index=idx`` with ``index=idx.copy(deep=True)`` doesn't help (something which would maybe require a fix on its own).

#### Expected Output
```python
In [5]: df.unstack(fill_value=3)
Out[5]: 
   0        1      
   A  B  C  A  B  C
a  1  1  1  0  0  0
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
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: 0.21.0.dev+572.g8e89cb3e1
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: 2.0.2
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>
