combine_first not retaining dtypes - unmatched indexes
```
df1 = pd.DataFrame([["one", i] for i in range(3)], columns=["a","b"])
df2 = pd.DataFrame([["one", i] for i in range(5)], columns=["a","b"])
df3 = df1.combine_first(df2)
df1.dtypes
df2.dtypes
df3.dtypes

#all below statements should show a dtype of int64 for column b
df1.dtypes
df2.dtypes
df3.dtypes

#Actual Output
df1.dtypes
a    object
b     int64
dtype: object
>>> df2.dtypes
a    object
b     int64
dtype: object
>>> df3.dtypes
a     object
b    float64
dtype: object

```
Not sure this is intended behavior or not, but as you can see the from the output the dtype of the col `b` is changed to `float64` when `combine_first` is called.  

I've seen an old open issue: [combine_first not retaining dtypes](https://github.com/pandas-dev/pandas/issues/7509)

That issue is from 2014 and explains why data types are coerced to `float64` when there is a resulting `nan`, however in the example above there is no "resulting" `nan`.  It could be because "under the hood" there are `nan`s where the first index doesn't match the second index.  Still this sort of leaves combine_first in a weird state because if i can't trust dtypes to not be coerced when appending data, then I need to guarantee matching indexes before hand.  If i have to do that, I sort of have to do half of the work of combine_first manually, making it far less useful.

#### Expected Output
```
df1.dtypes
a    object
b     int64
dtype: object
>>> df2.dtypes
a    object
b     int64
dtype: object
>>> df3.dtypes
a     object
b    int64
dtype: object
```

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

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.0.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.4
pytest: None
pip: 18.1
setuptools: 39.0.1
Cython: None
numpy: 1.15.1
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
```

</details>
