Concatenating rows with Int64 datatype coerces to object
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

df_a = pd.DataFrame({'a': [-1, -1, -1]})
df_b = pd.DataFrame({'b': [1, 1, 1]})
df_a['a'] = df_a['a'].astype('Int64')
df_b['b'] = df_b['b'].astype('Int64')

total = pd.concat([df_a, df_b], ignore_index=True)

print(total.dtypes)
```

output:
```
a    object
b    object
dtype: object
```
#### Problem description
When running the exact same code with floats, the output is similar to the expected output. It also happens with the append function
```
import pandas as pd

df_a = pd.DataFrame({'a': [-1, -1, -1]})
df_b = pd.DataFrame({'b': [1, 1, 1]})
df_a['a'] = df_a['a'].astype('Int64')
df_b['b'] = df_b['b'].astype('Int64')

total = df_a.append([df_b], ignore_index=True)

print(total.dtypes)
```

#### Expected Output
```
a    Int64
b    Int64
dtype: object
```

#### Output of ``pd.show_versions()``
```
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.7.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-43-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.0.dev0+997.ga197837
pytest: None
pip: 10.0.1
setuptools: 39.1.0
Cython: 0.28.2
numpy: 1.15.4
scipy: 1.1.0
pyarrow: 0.9.0
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.2
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: None
numexpr: 2.6.8
feather: 0.4.0
matplotlib: 2.2.2
openpyxl: None
xlrd: 1.1.0
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
gcsfs: None
```