* Updated HISTORY.rst.
* Moved version pin for idna to allow recent 2.7 release.
* Updated version warning for urllib3 to allow 1.23.
* Updated metadata dunders in __version__.py.
* Removed Python 2.6 trove classifier.
* Removed Python 2.6 reference from README.rst.

Without it, the cookie policy would get lost while making a request.
That would be invisible to the user if the default cookie policy wasn't
changed, of course.

>>> from requests import post
>>> r = post("https://example.com", files={"file-name": None})

However, when a param value or json field is None they are not included in the request body.

>>> from requests import get
>>> r = get("https://example.com", params={"file-name": None})
>>> r.request.url

This commit makes the beahviour consistent for files.

- the Request class doesn't accept JSON, but a JSON serializable object