BUG: Nullable integer type cols become 'object' dtype by concatenation
#### Code Sample, a copy-pastable example if possible

```python
In [1]: import pandas as pd                                                                                                                                                                        

In [2]: foo = pd.DataFrame({'a': [1]}).astype('Int64')                                                                                                                                             

In [3]: bar = pd.DataFrame({'a': [2], 'b': [3]}).astype('Int64')                                                                                                                                   

In [4]: pd.concat((foo, bar), sort=False)                                                                                                                                                          
Out[4]: 
   a    b
0  1  NaN
0  2    3

In [5]: pd.concat((foo, bar), sort=False).dtypes                                                                                                                                                   
Out[5]: 
a     Int64
b    object
dtype: object

```
#### Problem description

As shown in the code above, `pd.concat(foo, bar)` adds column 'b' to `foo` by filling it NaN and stacks them.
In this time, I expect the column 'b' still hold `Int64` because it accepts NaN but current behavior is not.

#### Expected Output
```python
In [5]: pd.concat((foo, bar), sort=False).dtypes                                                                                                                                                   
Out[5]: 
a     Int64
b    Int64
dtype: object
```


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

<details>
In [2]: pd.show_versions()                                                                                                                                                                         

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.3
numpy            : 1.17.3
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.0.3
setuptools       : 40.8.0
Cython           : None
pytest           : 3.10.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 7.9.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
</details>
