pandas.DataFrame.astype casts numerical nulls to string 'nan'  

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

s = pd.Series([1, 2, np.nan])
print str(s.isnull().any())

s = s.astype(str)
print str(s.isnull().any())

##output
True 
False
```
#### Problem description

When casting a series from float to string, the nulls in float should also be nulls in string. But, astype method converts the nulls to string 'nan', and so it does not reflect as null.

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-154-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_IN
LOCALE: None.None

pandas: 0.24.1
pytest: 2.8.7
pip: 19.0.1
setuptools: 40.6.2
Cython: None
numpy: 1.16.1
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 5.5.0
sphinx: None
patsy: 0.5.1
dateutil: 2.7.5
pytz: 2018.9
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.4.3
feather: None
matplotlib: 1.5.1
openpyxl: 2.3.0
xlrd: 0.9.4
xlwt: 0.7.5
xlsxwriter: None
lxml.etree: 3.5.0
bs4: 4.4.1
html5lib: 0.999
sqlalchemy: None
pymysql: None
psycopg2: 2.7.3.2 (dt dec pq3 ext lo64)
jinja2: 2.8
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
