BUG: feather NaN data type issue
# Issue
Feather recasting nan to None instead
![image](https://user-images.githubusercontent.com/18221871/92077744-97533200-edef-11ea-8c8e-5ed8ed9d9991.png)
![image](https://user-images.githubusercontent.com/18221871/92077755-9b7f4f80-edef-11ea-94aa-d29b5cf7de62.png)


#### Code Sample, a copy-pastable example
```python
x = {"a": [np.nan , "xyz"]}
df = pd.DataFrame(x)
df

print(type(df['a'].values[0]))
print(df['a'].values[0])

df.to_feather("example.feather")
df_read = pd.read_feather("example.feather")
df_read

```




```python
x = {"a": [np.nan, "xyz"]}
df = pd.DataFrame(x)
df

print(type(df['a'].values[0]))
print(df['a'].values[0])

df.to_pickle("example.feather")
df_read = pd.read_pickle("example.feather")
df_read

```

#### Problem description

[this should explain **why** the current behaviour is a problem and why the expected output is a better solution]

#### Expected Output
expected to have NaN output

#### Output of ``pd.show_versions()``
<details>

```

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 126 Stepping 5, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.25.0
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.2.2
setuptools       : 41.4.0
Cython           : 0.29.13
pytest           : 5.4.1
hypothesis       : 5.8.1
sphinx           : 2.2.0
blosc            : None
feather          : None
xlsxwriter       : 1.2.1
lxml.etree       : None
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.18.1
pandas_datareader: None
bs4              : 4.8.0
bottleneck       : 1.2.1
fastparquet      : 0.3.2
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.3.0
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.5
pandas_gbq       : 0.13.2
pyarrow          : 0.17.1
pytables         : None
s3fs             : 0.4.0
scipy            : 1.5.2
sqlalchemy       : 1.3.9
tables           : 3.6.1
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.1


```
</details>
