BUG: Data types are not preserved while concatenating DataFrames with nullable integers
#### Copy-pastable example

``` python
# input data
import pandas as pd
t1 = pd.DataFrame(index=[0], data={'x':[1]}, dtype='UInt8')
t2 = pd.DataFrame(index=[1], data={'y':[1]}, dtype='UInt8')
t3 = pd.concat([t1,t2], join='outer', sort=False)

'''actual result'''
print(t3.dtypes)
# x    object
# y    object
# dtype: object
```

#### Problem description

Data types are not preserved data type while concatenating DataFrames with nullable integers. Instead, the result of concatenation has mixed data types and so the column types are `object`:

``` python
>>> type(t3.at[0,'x'])
<class 'int'>
>>> type(t3.at[1,'x'])
<class 'float'>
```

#### Expected Output

``` python
'''expected result'''
print(t3.dtypes)
# x    UInt8
# y    UInt8
# dtype: object
```

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

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 3.10.0-862.11.6.el7.x86_64
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : en_US.utf-8
LANG             : en_US.utf-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.0
numpy            : 1.16.3
pytz             : 2018.4
dateutil         : 2.7.3
pip              : 19.1.1
setuptools       : 39.0.1
Cython           : 0.29.12
pytest           : 3.3.2
hypothesis       : None
sphinx           : 1.6.6
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.3.4
html5lib         : 0.9999999
pymysql          : None
psycopg2         : None
jinja2           : 2.10
IPython          : 6.2.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.3.4
matplotlib       : 3.0.2
numexpr          : 2.6.4
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.11.1
pytables         : None
s3fs             : None
scipy            : 1.1.0
sqlalchemy       : None
tables           : 3.5.2
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
</details>
