[0.24.1] New nullable integer fillna with non-int doesn't coerce to object
#### Code Sample

```python
import pandas as pd

sample_data = []

sample_data.append({"integer_column":None})
sample_data.append({"integer_column":1})
sample_data.append({"integer_column":2})

df = pd.DataFrame(sample_data)

# Previous type is object
# df.dtypes

df.loc[:,'integer_column'] = df.loc[:,'integer_column'].astype('Int64')

# Check new type is Int64, nullable
# df.dtypes

df.fillna('null_string')

```
#### Problem description

Using the new nullable type Int64, it is not possible to fill "NaN" values with other value.

### Error raised

> TypeError: <U11 cannot be converted to an IntegerDtype


#### Expected Output

The new dataframe should have replaced it's NaN values with the desired input of .fillna() method.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 85 Stepping 4, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: None.None

pandas: 0.24.1
pytest: 3.3.2
pip: 9.0.1
setuptools: 38.4.0
Cython: 0.27.3
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.6
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.2
openpyxl: 2.5.12
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml.etree: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.1.18
pymysql: None
psycopg2: None
jinja2: 2.8.1
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None


</details>
