Series convert int64 to float64 even dtype is explicitly set
#### Code Sample, a copy-pastable example if possible

```python
s1 = pd.Series({'v1': 1}, dtype='int64')
# >>> s1
# v1    1
# dtype: int64
s2 = pd.Series({'v2': 1}, dtype='int64')
# >>> s2
# v2    1
# dtype: int64
s1.add(s2, fill_value=0)
# v1    1.0
# v2    1.0
# dtype: float64
```
#### Problem description
Not sure if it is similar to #20070, `Series` seems **always** infer dtype even `dtype` is explicitly set 
 (`int64` to `float64`, `datetime64` to `int64`).

#### Expected Output
Keep to `dtype` and don't infer it.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: zh_CN.UTF-8
LOCALE: zh_CN.UTF-8

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 38.5.2
Cython: None
numpy: 1.14.1
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.0
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: None
sqlalchemy: None
pymysql: 0.7.11.None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
