MultiIndex.from_product() casts float to int when corresponding int is also present
#### Code Sample, a copy-pastable example if possible

```python
In [2]: pd.MultiIndex.from_product([pd.Index([1, 1.], dtype=object), range(3)]).values
Out[2]: array([(1, 0), (1, 1), (1, 2), (1, 0), (1, 1), (1, 2)], dtype=object)

In [3]: pd.MultiIndex(levels=[pd.Index([1, 1.], dtype=object), range(3)], labels=[[0,1], [1,1]]
   ...: )
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-3-a2ad432a7b0f> in <module>()
----> 1 pd.MultiIndex(levels=[pd.Index([1, 1.], dtype=object), range(3)], labels=[[0,1], [1,1]])

/home/nobackup/repo/pandas/pandas/core/indexes/multi.py in __new__(cls, levels, labels, sortorder, names, copy, verify_integrity, _set_identity, name, **kwargs)
    238 
    239         if verify_integrity:
--> 240             result._verify_integrity()
    241         if _set_identity:
    242             result._reset_identity()

/home/nobackup/repo/pandas/pandas/core/indexes/multi.py in _verify_integrity(self, labels, levels)
    281                                  "level {level}".format(
    282                                      values=[value for value in level],
--> 283                                      level=i))
    284 
    285     @property

ValueError: Level values must be unique: [1, 1.0] on level 0

In [4]: pd.Index([1, 1.], dtype=object)
Out[4]: Index([1, 1.0], dtype='object')
```

#### Problem

If a flat (object) ``Index`` allows us to distinguish 1 and 1., the same should do ``MultiIndex``. From https://github.com/pandas-dev/pandas/issues/18913#issuecomment-353748642

#### Expected Output

Both the first two examples should return a ``MultiIndex`` containing both 1 and 1.0 in its first level.

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

<details>

INSTALLED VERSIONS
------------------
commit: 8cbee356da1161c56c64f6f89cb5548bcadc3e44
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-5-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.23.0.dev0+182.g8cbee356d.dirty
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.7.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.3.0
xlsxwriter: 0.9.6
lxml: 4.1.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1


</details>
