Fix #1322: Add note in docs about None not being sent as data

Fix #1322: Add note in docs about None not being sent as data

In the case:

  payload = {'key1': 'value1', 'key2': 'value2', 'key3': None}
  r = requests.get("http://httpbin.org", params=payload)

the parameter `key3` will not be sent as a parameter in the URL.

Mention this in the documentation.