Using to_json() while reading from a specific csv file causes Django to not store the variable in the session
Inside of a Django view:

```python
    my_names = ['Date','Open','High','Low','Close','Volume','B0','B1','B2','B3','B4','B5','B6','B7']
    request.session['df'] = pd.read_csv( request.FILES.get('data'), names=my_names ).to_json()
    if request.session.test_cookie_worked():
        print('Test cookie worked! Deleting... ')
        request.session.delete_test_cookie()
    else:
        request.session.set_test_cookie()
        print('set test cookie')

```
#### Problem description

If I comment out the line "request.session['df'] = pd.read_csv( request.FILES.get('data'), names=my_names ).to_json()", then the first page load will print "set test cookie" on the first page load. On the second page load, the link 'Test cookie worked! Deleting..." will appear.

When I run the code as-is, the Terminal prints set test cookie on every single page load.

_GitHub won't let me upload the csv file even though it's zipped. Let me know how I can send that to you._

#### Expected Output

The terminal output should alternate between 'set test cookie' and 'Test cookie worked!' on every page load.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-51-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.1
nose: None
pip: 9.0.1
setuptools: 20.7.0
Cython: None
numpy: 1.11.2
scipy: None
statsmodels: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2016.7
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: 0.7.3
lxml: None
bs4: 4.4.1
html5lib: 0.999
httplib2: 0.9.1
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
None
</details>
