Small fix broken cookie parse

Code:
```python
import requests
resp = requests.get('http://www.servanegaxotte.com') 
# or khvigra.ru, nastersrl.ru, spcentre2000.ru etc.
```

Raised exception:
```
  File "C:\Python27\lib\cookielib.py", line 1645, in extract_cookies
    self.set_cookie(cookie)
  File "C:\Python27\lib\site-packages\requests\cookies.py", line 270, in set_cookie
    if cookie.value.startswith('"') and cookie.value.endswith('"'):
AttributeError: 'NoneType' object has no attribute 'startswith'
```