reindex(fill_value=None) fills with np.NaN instead of None
#### Code Sample, a copy-pastable example if possible

``` python
In [12]: import pandas as pd

In [13]: s = pd.Series(['a', 'b'])

In [14]: s.reindex([0,1,2], fill_value=None)
Out[14]: 
0      a
1      b
2    NaN
dtype: object
```
#### Expected Output

``` python
0      a
1      b
2    None
dtype: object
```
#### output of `pd.show_versions()`

``` ------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-34-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.0
nose: None
pip: 8.1.2
setuptools: 25.2.0
Cython: 0.20.1
numpy: 1.8.1
scipy: 0.17.0
statsmodels: 0.6.1
xarray: None
IPython: 1.1.0
sphinx: 1.2b1
patsy: 0.4.1
dateutil: 2.4.2
pytz: 2013b
blosc: None
bottleneck: None
tables: 3.1.1
numexpr: 2.4
matplotlib: None
openpyxl: 2.3.0
xlrd: 0.9.3
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 0.9.8
pymysql: None
psycopg2: None
jinja2: 2.7
boto: None
```
#### in source

This is happening in BlockManager.reindex_indexer()
https://github.com/pydata/pandas/blob/8af626474f6f314527a9ad3f15403aa2dd8c402d/pandas/core/internals.py#L3820-L3822
