dtype-unaware (empty) objects ("any" dtype)
#### Code Sample, a copy-pastable example if possible

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

In [3]: s.loc['myint'] = 1

In [4]: s.loc['myfloat'] = 2.

In [5]: s
Out[5]: 
myint      1.0
myfloat    2.0
dtype: float64
```
#### Problem description

When an empty Series is added the first object, it does inference on it and sets its dtype ( https://github.com/pandas-dev/pandas/issues/19576#issuecomment-363875752 ). This can be nice... except if the user had passed a specific dtype on construction.

Empty objects should (by default) have no dtype set (or have an ["Any" dtype](https://github.com/pandas-dev/pandas/issues/19576#issuecomment-364680239)), and inference should be done only in this case.

#### Expected Output

#### 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: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 33.1.1
Cython: 0.25.2
numpy: 1.13.3
scipy: 0.18.1
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: 1.4.9
patsy: 0.4.1+dev
dateutil: 2.5.3
pytz: 2016.7
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 2.0.0
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: None
xlsxwriter: 0.9.6
lxml: 3.7.1
bs4: 4.5.3
html5lib: 0.999999999
httplib2: 0.9.2
apiclient: None
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None


</details>
