Unintended cast to float when adding row with multiindex
I would have expected this to be already reported but couldn't find anything, sorry in advance if I missed it.

#### Code Sample, a copy-pastable example if possible

```python
In [2]: df = pd.DataFrame([[1,2], [3,4]], index=pd.MultiIndex.from_product([['a', 'b'], ['c']]))

In [3]: df.loc[('c', 'd'), :] = [5,6]

In [4]: df
Out[4]: 
       0    1
a c  1.0  2.0
b c  3.0  4.0
c d  5.0  6.0
```
#### Problem description

Casting to float wasn't expected nor required. By the way: ``df.loc(axis=0)[('c', 'd')] = [5,6]`` behaves the same (so it is not a matter of the "total" slice not being identified as such).

#### Expected Output

```python
n [5]: df.astype(int)
Out[5]: 
     0  1
a c  1  2
b c  3  4
c d  5  6
```

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

<details>
INSTALLED VERSIONS
------------------
commit: 9e7666dae3b3b10d987ce154a51c78bcee6e0728
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+265.g9e7666dae
pytest: 3.0.6
pip: 9.0.1
setuptools: None
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
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
pandas_gbq: None
pandas_datareader: 0.2.1
</details>
