read_csv() will throw a exception when csv file has str 'NaN'
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
from bokeh.charts import Bar, Line
from bokeh.plotting import output_file,show

output_file("test.html")

wt = pd.read_csv('test.csv', dtype={
    'word': str,
    'times': int,
})

print(wt['word'])
line = Line(wt,
            x='word',
            y='times'
            )
show(line)

```

The test.csv file showed below:
```csv
word,times
nan,1
```
#### Problem description
read_csv() will throw a exception when read csv file has str 'NaN'

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

I think it has a str 'nan' becuase it conflicts with python built-in words.

if i remove 'nan' string, it works good.
#### Output
```

```

#### Expected Output


#### Output of ``pd.show_versions()``
<details>
# Paste the output here pd.show_versions() here
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.8.0-2-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: 
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.2
nose: None
pip: 9.0.1
setuptools: 32.3.0
Cython: None
numpy: 1.12.0
scipy: None
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 2.0.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.5.1
html5lib: 0.999999999
httplib2: 0.9.2
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.4
boto: None
pandas_datareader: None

</details>
