Adding element to empty object Series changes dtype
#### Code Sample, a copy-pastable example if possible

```python
In [2]: s = pd.Series(dtype='object')

In [3]: s.loc[0] = 1

In [4]: s
Out[4]: 
0    1
dtype: int64

In [5]: s = pd.Series(dtype='object')

In [6]: s.loc[0] = 1.

In [7]: s
Out[7]: 
0    1.0
dtype: float64
```
Compare with:
```python

In [8]: s = pd.Series({1 : 2}, dtype='object')

In [9]: s.loc[0] = 1

In [10]: s
Out[10]: 
1    2
0    1
dtype: object

In [11]: s = pd.Series({1 : 2.}, dtype='object')

In [12]: s.loc[0] = 1.

In [13]: s
Out[13]: 
1    2
0    1
dtype: object
```

#### Problem description

The dtype of a ``Series`` should change, on addition of new elements, only when it has to:: as a corollary, it should never change when it is ``object``.

Vaguely related to #17261, maybe to #8902.

#### Expected Output

Always ``object`` dtype.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-5-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+229.gd5a7e7c94
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>
