Actual dtypes of pd.DataFrame(None, [...], dtype=int)
#### Code Sample, a copy-pastable example if possible

```python
In [2]: pd.DataFrame(index=range(2), columns=['a', 'b'], dtype=int).dtypes
Out[2]: 
a    object
b    object
dtype: object
```
compare to
```python
In [3]: pd.Series(index=range(2), dtype=int).dtype
Out[3]: dtype('float64')
```
#### Problem description

See https://github.com/pandas-dev/pandas/pull/18600#r167405218

Wrong test in https://github.com/pandas-dev/pandas/blob/324379ce75269aa6bced90ecf3edb692539a2742/pandas/tests/frame/test_constructors.py#L738
 
Fixed by #18600

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

<details>
INSTALLED VERSIONS
------------------
commit: 50528421ae79b27a26b32ba715b17271c8dfda7e
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+248.g50528421a
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>
