'Do not send headers in Session.headers with None values.'

def test_headers_on_session_with_None_are_not_sent(self, httpbin):

    ses = requests.Session()
    ses.headers['Accept-Encoding'] = None
    req = requests.Request('GET', httpbin('get'))
    prep = ses.prepare_request(req)
    assert ('Accept-Encoding' not in prep.headers)
