BUG: fillna maximum recursion depth exceeded in cmp
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import numpy as np

s = pd.Series([float('nan')])
t = s.fillna(2 ** 63)         # RuntimeError: maximum recursion depth exceeded in cmp
t = s.fillna(float(2 ** 63))  # works
```
#### Problem description

A `fillna` call with a large integer causes infinite recursion. Converting to a float before passing it in seems to fix it. Maybe the arg of `fillna` should be converted to the series dtype immediately?

Note that this bug only affects PY2, PY3 works just fine.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.14.final.0
python-bits: 64
OS: Linux
OS-release: 3.x.x...x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.21.0
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.6.0
Cython: 0.27.3
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 5.4.1
sphinx: None
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: 1.1.15
pymysql: None
psycopg2: 2.7.3.2 (dt dec pq3 ext)
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.5.0

</details>
