The docs just read a bit strange with `decode_unicode=None` as opposed to `False`.

Like we just did for 'data', 'params' follows the same processing and
can also be a list of tuples.

The documentation for the 'data' parameter of various functions was
inconsistant. The low-level function, used by every convenience API,
internally uses RequestEncodingMixin._encode_params which accepts list
of tuples as well as dictionaries, hence every function taking a data
parameter also accepts a list of tuple.

Keep the documentation short, but provide a link to Request for
details.

>>> 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.